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1. Introduction 

Welcome, gentle reader.

I have written a number of networking HOWTOs in the past, and it occurred to me that there's a hell of pileof jargon in each one. I had three choices: my other two were ignoring the problem and explaining the termseverywhere. Neither was attractive.

The point of Free software is that you should have the freedom to explore and play with the software systemyou use. I believe that enabling people to experience this freedom is a noble goal; not only do people feelempowered by the pursuit (such as rebuilding a car engine) but the nature of the modern Internet and Freesoftware allows you to share the experience with millions.

But you have to start somewhere, so here we are.

(C) 2000 Paul `Rusty' Russell. Licenced under the GNU GPL.

2. What is a `computer network'? 

A computer network is just a set of stuff for nodes to talk to each other (by `nodes' I mean computers, printers, Coke machines and whatever else you want). It doesn't really matter how they are connected: theycould use fiber-optic cables or carrier pigeons. Obviously, some choices are better than others (especially if you have a cat).

Usually if you just connect two computers together, it's not called a network; you really need three or more t become a network. This is a bit like the word `group': two people is just a couple of guys, but three can be a`group'. Also, networks are often hooked together, to make bigger networks; each little network (usuallycalled a `sub-network') can be part of a larger network.

The actual connection between two computers is often called a `network link'. If there's a bit of cable runninout of the back of your machine to the other machines, that's your network link.

There are four things which we usually care about when we talk about a computer network:

Size 

If you simply connect your four computers at home together, you have what is called a LAN (LocalArea Network). If everything is within walking distance, it's usually called a LAN, however manymachines are connected to it, and whatever you've built the network out of.

The other end of the spectrum is a WAN (Wide Area Network). If you have one computer in Lahore,Pakistan, one in Birmingham, UK and one in Santiago, Chile, and you manage to connect them, it's WAN.

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Topology: The Shape 

Draw a map of the network: lines are the

network links, and each node is a dot. Maybe each line leads into a central node like a big star,meaning that everyone talks through one point (a `star topology'):

o o o \_ | _/ \|/ o-----o-----o _/|\_ / | \ o o o 

Maybe everyone talks in a line, like so:

o------o------o-------o--------o | | | | | o | | o | o 

Or maybe you have three subnetworks connected through one node:

o o | o--o--o | | | o--o--o--o--o o \ | o------o 

/ | o--o--o--o--o o | | | o | o--o o 

You'll see many topologies like these in real life, and many far more complicated.

Physical: What It's Made Of  

The second thing to care about is what you've built the network out of. The cheapest is `sneakernet',where badly-dressed people carry floppy disks from one machine to the others. Sneakernet is almost

always a LAN. Floppies cost less than $1, and a solid pair of sneakers can be got for around $20.

The most common system used to connect home networks to far bigger networks is called a `modem(for MODulator/DEModulator), which turns a normal phone connection into a network link. It turnsthe stuff the computer sends into sounds, and listens to sounds coming from the other end to turnthem back into stuff for the computer. As you can imagine, this isn't very efficient, and phone linesweren't designed for this use, but it's popular because phone lines are so common and cheap: modemsell for less than $50, and phone lines usually cost a couple of hundred dollars a year.

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The most common way to connect machines into a LAN is to use Ethernet. Ethernet comes in thesemain flavors (listed from oldest to newest): Thinwire/Coax/10base2, UTP (Unshielded TwistedPair)/10baseT and UTP/100baseT. Gigabit ethernet (the name 1000baseT is starting to get silly) isstarting to be deployed, too. 10base2 wire is usually black coaxial cable, with twist-on T-pieces toconnect them to things: everyone gets connected in a big line, with special `terminator' pieces on thetwo ends. UTP is usually blue wire, with clear `click-in' phone-style connectors which plug intosockets to connect: each wire connects one node to a central `hub'. The cable is a couple of dollars ameter, and the 10baseT/10base2 cards (many cards have plugs for both) are hard to get brand new.100baseT cards, which can also speak 10baseT as well, are ten times faster, and about $30.

On the other end of the spectrum is Fiber; a continuous tiny glass filament wrapped in protectivecoating which can be used to run between continents. Generally, fiber costs thousands.

We usually call each connection to a node a `network interface', or `interface' for short. Linux givesthese names like `eth0' for the first ethernet interface, and `fddi0' for the first fiber interface. The`/sbin/ifconfig' command lists them.

Protocol: What It's Speaking 

The final thing to care about is the language the two are speaking. When two modems are talking toeach other down a phone line, they need to agree what the different sounds mean, otherwise it simplywon't work. This convention is called a `protocol'. As people discovered new ways of encoding whatthe computer says into smaller sounds, new protocols were invented; there are at least a dozendifferent modem protocols, and most modems will try a number of them until they find one the otherend understands.

Another example is the 100baseT network mentioned above: it uses the same physical network linksUTP) as 10baseT above, but talks ten times as fast.

These two protocols are what are called `link-level' protocols; how stuff is handed over the individuanetwork links, or `one hop'. The word `protocol' also refers to other conventions which are followed,as we will see next.

3. What is the `Internet'? 

The Internet is a WAN which spans the entire globe: it is the largest computer network in existence. The

 phrase `internetworking' refers to connecting separate networks to build a larger one, hence `The Internet' isthe connection of a whole pile of subnetworks.

So now we look at the list above and ask ourselves: what is the Internet's size, physical details and protocols

The size is already established above: it's global.

The physical details are varied however: each little sub-network is connected differently, with a different

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appropriate link layer, which transmits it to the other end of the link.

At the other end, the link layer hands it up to the IP layer, which sees it is destined for this host (if not, itmight hand it down to a different link layer to go out to the next node), hands it up to the TCP layer, whichhands it to the server.

So we have the following breakdown:

1. The application (Netscape, or the web server at the other end) decides who it wants to talk to, andwhat it wants to send).

2. The TCP layer sends special packets to start the conversation with the other end, and then packs thedata into a TCP `packet': a packet is just a term for a chunk of data which passes through a network.The TCP layer hands this packet to the IP layer: it then keeps sending it to the IP layer until the TCPlayer at the other end replies to say that it has received it. This is called `retransmission', and has awhole heap of complex rules which control when to retransmit, how long to wait, etc. It also giveseach packet a set of numbers, which mean that the other end can sort them into the right order.

3. The IP layer looks at the destination of the packet, and figures out the next node to send the packet toThis simple act is called `routing', and ranges from really simple (if you only have one modem, andno other network interfaces, all packets should go out that interface) to extremely complex (if youhave 15 major networks connected directly to you).

4. This IP Thing 

So the role of the IP layer is to figure out how to `route' packets to their final destination. To make this possible, every interface on the network needs an `IP address'. An IP address consists of four numbersseparated by periods, like `167.216.245.249'. Each number is between zero and 255.

Interfaces in the same network tend to have neighboring IP addresses. For example, `167.216.245.250' sitsright next to the machine with the IP address `167.216.245.249'. Remember also that a router is a node withinterfaces on more than one network, so the router will have one IP address for each interface.

So the Linux Kernel's IP layer keeps a table of different `routes', describing how to get to various groups of IP addresses. The simplest of these is called a `default route': if the IP layer doesn't know better, this is wherit will send a packet onwards to. You can see a list of routes using `/sbin/route'.

Routes can either point to a link, or a particular node which is connected to another network. For example,when you dial up to the ISP, your default route will point to the modem link, because that's where the entireworld is.

Rusty's ISP's ~~~~~~~~~~~~Modem Modem { } o------------------o { The Internet } { } ~~~~~~~~~~~~

But if you have a permanent machine on your network which connects to the outside world, it's a bit more

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complicated. In the diagram below, my machine can talk directly to Tridge and Paul's machines, and to thefirewall, but it needs to know that packets heading the rest of the world need to go to the firewall, which wil pass them on. This means that you have two routes: one which says `if it's on my network, just send itstraight there' and then a default route which says `otherwise, send it to the firewall'.

o Tridge's | Work Machine ~~~~~~~~~~~~ Rusty's | { }Work Machine o--------+-----------------o--{ The Internet } 

| Firewall { }| ~~~~~~~~~~~~ o Paul's Work Machine 

4.1 Groups of IP Addresses: Network Masks 

There is one last detail: there is a standard notation for groups of IP addresses, sometimes called a `network address'. Just like a phone number can be broken up into an area prefix and the rest, we can divide an IPaddress into a network prefix and the rest.

It used to be that people would talk about `the 1.2.3 network', meaning all 256 addresses from 1.2.3.0 to1.2.3.255. Or if that wasn't a big enough network, they might talk about the `1.2 network' which meant alladdresses from 1.2.0.0 to 1.2.255.255.

We usually don't write `1.2.0.0 - 1.2.255.255'. Instead, we shorten it to `1.2.0.0/16'. This weird `/16' notation(it's called a `netmask') requires a little explanation.

Each number between the dots in an IP address is actually 8 binary digits (00000000 to 11111111): we write

them in decimal form to make it more readable for humans. The `/16' means that the first 16 binary digits isthe network address, in other words, the `1.2.' part is the the network (remember: each digit represents 8 binary digits). This means any IP address beginning with `1.2.' is part of the network: `1.2.3.4' and `1.2.3.50are, and `1.3.1.1' is not.

To make life easier, we usually use networks ending in `/8', `/16' and `/24'. For example, `10.0.0.0/8' is a bignetwork containing any address from 10.0.0.0 to 10.255.255.255 (over 16 million addresses!). 10.0.0.0/16 ismaller, containing only IP addresses from 10.0.0.0 to 10.0.255.255. 10.0.0.0/24 is smaller still, containingaddresses 10.0.0.0 to 10.0.0.255.

To make things confusing, there is another way of writing netmasks. We can write them like IP addresses:

10.0.0.0/255.0.0.0 

Finally, it's worth noting that the very highest IP address in any network is reserved as the `broadcastaddress', which can be used to send a message to everyone on the network at once.

Here is a table of network masks:

Short Full Maximum Comment 

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Form Form #Machines 

/8 /255.0.0.0 16,777,215 Used to be called an `A-class' /16 /255.255.0.0 65,535 Used to be called an `B-class' /17 /255.255.128.0 32,767 /18 /255.255.192.0 16,383 /19 /255.255.224.0 8,191 /20 /255.255.240.0 4,095 /21 /255.255.248.0 2,047 /22 /255.255.252.0 1,023 

/23 /255.255.254.0 511 /24 /255.255.255.0 255 Used to be called a `C-class' /25 /255.255.255.128 127 /26 /255.255.255.192 63 /27 /255.255.255.224 31 /28 /255.255.255.240 15 /29 /255.255.255.248 7 /30 /255.255.255.252 3 

5. Machine Names and IP Addresses 

So every interface on every node has an IP address. It was realized quite quickly that humans are pretty badat remembering numbers, so it was decided (just like phone numbers) to have a directory of names. But sincwe're using computers anyway, it's nicer to have the computer look up the names for us automatically.

Hence we have the Domain Name System (DNS). There are nodes with well known IP addresses which programs can ask to look up names, and return IP addresses. Almost all programs you will use are capable odoing this, which is why you can put `www.linuxcare.com' into Netscape, instead of `167.216.245.249'.

Of course, you need the IP address of at least one of these `name servers': usually these are kept in the`/etc/resolv.conf' file.

Since DNS queries and responses are fairly small (1 packet each), the TCP protocol is not usually used: it provides automatic retransmission, ordering and general reliability, but at a cost of sending extra packetsthrough the network. Instead we use the very simple `User Datagram Protocol', which doesn't offer any of thfancy TCP features we don't need.

6. Different Services: Email, Web, FTP, Name Serving 

In the earlier example, we showed Netscape sending a TCP request to a web server running on another nodeBut imagine that the node with the web server is also running an Email server, an FTP server and a nameserver: how does it know which server the TCP connection is for?

This is where TCP and UDP have a concept of `ports'. Every packet has space for a `destination port', whichsays what service the packet is for. For example, TCP port 25 is the mail server, and TCP port 80 is the webserver (although sometimes you find web servers on different ports). A list of ports can be found in`/etc/services'.

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Also, if two Netscape windows are both accessing different parts of the same web site, how does the Linux box running Netscape sort out the TCP packets coming back from the web server?

This is where the `source port' comes in: every new TCP connection gets a different source port, so everyoncan tell them apart, even if they are going to the same destination IP address and the same destination port.Usually the first source port given will be 1024, and will increase over time.

7. Dialup Interfaces: PPP 

When you dial your modem to an ISP, and it connects to their modem, the kernel doesn't just shove IP packets through it. There is a protocol called `Point-to-Point Protocol', or `PPP', which is used to negotiatewith the other end before any packets are allowed through. This is used by the ISP to identify who is dialedup: on your Linux box, a program called the `PPP daemon' handles your end of the negotiation.

Because there are so many dialup users in the world, they usually don't have their own IP address: most ISPwill assign you one of theirs temporarily when you dial up (the PPP daemon will negotiate this). This is oftecalled a `dynamic IP address', as separate from a `static IP address' which is the normal case where you haveyour own address permanently. Usually they are assigned by modem: the next time you dial up, you will probably get a different modem in the modem pool, and hence a different IP address.

8. What Packets Look Like 

For the exceptionally curious (and the curiously exceptional), here is a description of what a packet actuallylooks like. There are several tools which watch what packets are passing in and out of your Linux box: themost common one is `tcpdump' (which understands more than TCP these days), but a nicer one is `ethereal'.Such programs are known as `packet sniffers'.

The start of each packet says where it's going, where it came from, the type of the packet, and other administrative details. This part is called the `packet header'. The rest of the packet, containing the actual da being transmitted, is usually called the `packet body'.

So any IP packet begins with an `IP header': at least 20 bytes long. It looks like (this diagram stolenshamelessly from RFC 791):

.-------+-------+---------------+-------------------------------.  |Version| IHL |Type of Service| Total Length | |-------+-------+---------------+-------------------------------|  | Identification |Flags| Fragment Offset | |---------------+---------------+-------------------------------|  | Time to Live | Protocol | Header Checksum | |---------------+---------------+-------------------------------|  | Source Address | |---------------------------------------------------------------|  | Destination Address | `---------------------------------------------------------------'  

The important fields are the Protocol, which indicates whether this is a TCP packet (number 6), a UDP pack

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(number 17) or something else, the Source IP Address, and the Destination IP Address.

 Now, if the protocol fields says this is a TCP packet, then a TCP header will immediately follow this IPheader: the TCP header is also at least 20 bytes long:

.-------------------------------+-------------------------------.  | Source Port | Destination Port | |-------------------------------+-------------------------------|  

| Sequence Number | |---------------------------------------------------------------|  | Acknowledgment Number | |-------------------+-+-+-+-+-+-+-------------------------------|  | Data | |U|A|P|R|S|F| | | Offset| Reserved |R|C|S|S|Y|I| Window | | | |G|K|H|T|N|N| | |-------+-----------+-+-+-+-+-+-+-------------------------------|  | Checksum | Urgent Pointer | `---------------------------------------------------------------'  

The most important fields here are the source port, and destination port, which says which service the packeis going to (or coming from, in the case of reply packets). The sequence and acknowledgement numbers are

used to keep packets in order, and tell the other end what packets have been received. The ACK, SYN, RSTand FIN flags (written downwards) are single bits which are used to negotiate the opening (SYN) and closin(RST or FIN) of connections.

Following this header comes the actual message which the application sent (the packet body). A normal packet is up to 1500 bytes: this means that the most space the data can take up is 1460 bytes (20 bytes for thIP header, and 20 for the TCP header): over 97%.

Summary  

So the modern Internet uses IP packets to communicate, and most of these IP packets use TCP inside. Specinodes called `routers' connect all the little networks together into larger networks, and pass these packetsthrough to their destination. Most normal machines are only attached to one network (ie. have only oneinterface), and so are not routers.

Every interface has a unique IP address, which look like `1.2.3.4': interfaces in the same network will haverelated IP addresses, with the same start, the same way that phone connections in the same area have thesame prefix. These network addresses look like IP addresses, with a `/' to say how much of them is the prefix

eg `1.2.0.0/16' means the first two digits is the network address: each digit represents 8 bits.

Machines are given names by the Domain Name Service: programs ask name servers to give them the IPaddress, given a name like `www.linuxcare.com'. This IP address is then used as the destination IP address ttalk to that node.

Rusty is really bad at writing documentation, especially for beginners.

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Enjoy!

Rusty.

Learning Path: Basic Network Concepts 

BASIC NETWORK CONCEPTS...circuit - network  - node - topology - bus - ring - campus - backbone -  bridge - switch - packet - circuit-switched -  packet-switched - frame - virtual circuit - local area network  - wide area network  - metropolitan arenework - duplex - gateway - giant - runt - Quality of Service (QoS) - network operations center  (NOC) - BER  

Circuit 

1) In electronics, a circuit is a path between two or more points along which an electrical current can be carried. (Acircuit breaker is a device that interrupts the path when necessary to protect other devices attached to the circuit - for example, in case of a power surge.)

2) In telecommunications, a circuit is a discrete (specific) path between two or more points along whichsignals can be carried. Unless otherwise qualified, a circuit is a physical path, consisting of one or more wire(or wireless paths) and possibly intermediate switching points. A network is an arrangement of circuits. In adial-up (switched) connection, a circuit is reserved for use by one user for the duration of the calling session.In a dedicated or leased line arrangement, a circuit is reserved in advance and can only be used by the owneor renter of the circuit.

A virtual circuit, sometimes called a logical circuit , is a path between two or more points that seems like a fixe physical path, but actually is one path out of many possible physical paths that can be arranged. A permanenvirtual circuit(PVC) is a virtual circuit that provides a guaranteed connection between two or more points

when needed without having to reserve or commit to a specific physical path in advance. This allows manycompanies to share a common pool of circuits. This approach is used in a frame relay network and offers acommitted set of resources to a telephone company customer at a lower price than if the customer leases theown circuits. A switched virtual circuit (SVC) is similar to a permanent virtual circuit, but allows users to diain to the network of virtual circuits.

Network 

In information technology, a network is a series of points or nodes interconnected by communication paths. Networkscan interconnect with other networks and contain subnetworks.

The most common topology or general configurations of networks include the  bus, star, Token Ring, and meshtopologies. Networks can also be characterized in terms of spatial distance as local area networks (LANs),metropolitan area networks (MANs), and wide area networks (WANs).

A given network can also be characterized by the type of data transmission technology in use on it (for example, a TCP/IP or Systems Network Architecture network); by whether it carries voice, data, or both kinds of signals; by who can use the network (public or private); by the usual nature of its connections (dial-up or switched, dedicated or nonswitched, or virtual connections); and by the types of physical links (for exampleoptical fiber , coaxial cable, and Unshielded Twisted Pair ). Large telephone networks and networks using their 

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infrastructure (such as the Internet) have sharing and exchange arrangements with other companies so thatlarger networks are created.

Node 

In a network, a node is a connection point, either a redistribution point or an end point for data transmissions. Ingeneral, a node has programmed or engineered capability to recognize and process or forward transmissions to othe

nodes. 

Topology 

A topology (from Greek topos meaning place) is a description of any kind of locality in terms of its layout. Incommunication networks, a topology is a usually schematic description of the arrangement of a network, including itsnodes and connecting lines. There are two ways of defining network geometry: the physical topology and the logical (signal) topology.

The physical topology of a network is the actual geometric layout of workstations. There are several commo

 physical topologies, as described below and as shown in the illustration.

In the  bus network topology, every workstation is connected to a main cable called the bus. Therefore, in effect,each workstation is directly connected to every other workstation in the network.

In the star network topology, there is a central computer or server to which all the workstations are directlyconnected. Every workstation is indirectly connected to every other through the central computer.

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In the ring network topology, the workstations are connected in a closed loop configuration. Adjacent pairs of workstations are directly connected. Other pairs of workstations are indirectly connected, the data passingthrough one or more intermediate nodes.

A variation of the ring network topology is known as the Token Ring. In this scheme, the signal travels in onlyone direction or senses around the ring, carried by a so-called token from node to node.

The mesh network topology employs either of two schemes, called full mesh and partial mesh. In the full meshtopology, each workstation is connected directly to each of the others. In the partial mesh topology, someworkstations are connected to all the others, and some are connected only to those other nodes with whichthey exchange the most data.

The tree network topology uses two or more star networks connected together. The central computers of thestar networks are connected to a main bus. Thus, a tree network is a bus network of star networks.

Logical (or signal) topology refers to the nature of the paths the signals follow from node to node. In manyinstances, the logical topology is the same as the physical topology. But this is not always the case. For example, some networks are physically laid out in a star configuration, but they operate logically as bus or ring networks.

Bus 

In a computer or on a network, a bus is a transmission path on which signals are dropped off or picked up at everydevice attached to the line. Only devices addressed by the signals pay attention to them; the others discard the signaAccording to Winn L. Rosch, the term derives from its similarity to autobuses that stop at every town or block to drop oor take on riders

In general, the term is used in two somewhat different contexts:

(1) A bus is a network topology or circuit arrangement in which all devices are attached to a line directly andall signals pass through each of the devices. Each device has a unique identity and can recognize thosesignals intended for it.

(2) In a computer, a bus is the data path on the computer's motherboard that interconnects the microprocessor with attachments to the motherboard in expansion slots (such as hard disk drives, CD-ROM drives, and

graphics adapters).

Ring 

Also see Web ring.

A ring is a network topology or circuit arrangement in which each device is attached along the same signal path to two other devices, forming a path in the shape of a ring. Each device in the ring has a unique address

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Information flow is unidirectional and a controlling device intercepts and manages the flow to and from thering. The Token Ring is the most prevalent form of ring network.

Campus 

In telecommunications, a campus is a physically contiguous association of locations such as several adjacent officebuildings. Typically, such areas require one or more local area networks and bridging, routing, and aggregation

equipment based on situational needs.

Backbone 

A backbone is a larger transmission line that carries data gathered from smaller lines that interconnect with it.

1) At the local level, a backbone is a line or set of lines that local area networks connect to for a wide area networkconnection or within a local area network to span distances efficiently (for example, between buildings).

2) On the Internet or other wide area network, a backbone is a set of paths that local or regional networks connect tofor long-distance interconnection. The connection points are known as network nodes or telecommunication dataswitching exchanges (DSEs). 

Bridge 

In telecommunication networks, a bridge is a product that connects a local area network (LAN) to another local areanetwork that uses the same protocol (for example, Ethernet or Token Ring). You can envision a bridge as being a devicethat decides whether a message from you to someone else is going to the local area network in your building or tosomeone on the local area network in the building across the street. A bridge examines each message on a LAN,"passing" those known to be within the same LAN, and forwarding those known to be on the other interconnected LAN

(or LANs).

In bridging networks, computer or node addresses have no specific relationship to location. For this reason,messages are sent out to every address on the network and accepted only by the intended destination node.Bridges learn which addresses are on which network and develop a learning table so that subsequentmessages can be forwarded to the right network.

Bridging networks are generally always interconnected local area networks since broadcasting every messagto all possible destinations would flood a larger network with unnecessary traffic. For this reason, router networks such as the Internet use a scheme that assigns addresses to nodes so that a message or packet can b

forwarded only in one general direction rather than forwarded in all directions.

A bridge works at the data-link (physical network) level of a network, copying a data frame from one networto the next network along the communications path.

A bridge is sometimes combined with a router in a product called a  brouter .

Switch 

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In a telecommunications network, a switch is a device that channels incoming data from any of multipleinput ports to the specific output port that will take the data toward its intended destination. In the traditionacircuit-switched telephone network, one or more switches are used to set up a dedicated though temporaryconnection or circuit for an exchange between two or more parties. On an Ethernet local area network (LAN), aswitch determines from the physical device (Media Access Control or MAC) address in each incomingmessage frame which output port to forward it to and out of. In a wide area  packet-switched network such as thInternet, a switch determines from the IP address in each  packet which output port to use for the next part of itstrip to the intended destination.

In the Open Systems Interconnection (OSI) communications model, a switch performs the layer 2 or Data-Link

layer function. That is, it simply looks at each packet or data unit and determines from a physical address (th"MAC address") which device a data unit is intended for and switches it out toward that device. However, inwide area networks such as the Internet, the destination address requires a look-up in a routing table by adevice known as a router. Some newer switches also perform routing functions (layer 3 or the  Network layer  

functions in OSI) and are sometimes called IP switches.

On larger networks, the trip from one switch point to another in the network is called a hop. The time a switctakes to figure out where to forward a data unit is called its latency. The price paid for having the flexibilitythat switches provide in a network is this latency. Switches are found at the backbone and gateway levels of anetwork where one network connects with another and at the subnetwork level where data is being forwardeclose to its destination or origin. The former are often known as core switches and the latter as desktop

 switches.

In the simplest networks, a switch is not required for messages that are sent and received within the networkFor example, a local area network may be organized in a Token Ring or   bus arrangement in which each possibdestination inspects each message and reads any message with its address.

Circuit-Switching version Packet-Switching  

A network's paths can be used exclusively for a certain duration by two or more parties and then switched for use toanother set of parties. This type of "switching" is known as circuit-switching and is really a dedicated and continuouslyconnected path for its duration. Today, an ordinary voice phone call generally uses circuit-switching.

Most data today is sent, using digital signals, over networks that use packet-switching . Using packet-switching, all network users can share the same paths at the same time and the particular route a data unittravels can be varied as conditions change. In packet-switching, a message is divided into packets, which areunits of a certain number of bytes. The network addresses of the sender and of the destination are added tothe packet. Each network point looks at the packet to see where to send it next. Packets in the same message

may travel different routes and may not arrive in the same order that they were sent. At the destination, the packets in a message are collected and reassembled into the original message.

Packet 

A packet is the unit of data that is routed between an origin and a destination on the Internet or any other  packet-switched network. When any file (e-mail message, HTML file, Graphics Interchange Format file, Uniform Resource Locatorrequest, and so forth) is sent from one place to another on the Internet, the Transmission Control Protocol (TCP) layerof TCP/IP divides the file into "chunks" of an efficient size for routing. Each of these packets is separately numberedand includes the Internet address of the destination. The individual packets for a given file may travel different routesthrough the Internet. When they have all arrived, they are reassembled into the original file (by the TCP layer at the

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receiving end).

A packet-switching scheme is an efficient way to handle transmissions on a connectionless network such asthe Internet. An alternative scheme, circuit-switched, is used for networks allocated for voice connections. Incircuit-switching, lines in the network are shared among many users as with packet-switching, but eachconnection requires the dedication of a particular path for the duration of the connection.

"Packet" and "datagram" are similar in meaning. A protocol similar to TCP, the User Datagram Protocol(UD

uses the term datagram. 

Circuit-switched 

Circuit-switched is a type of network in which a physical path is obtained for and dedicated to a single connectionbetween two end-points in the network for the duration of the connection. Ordinary voice phone service is circuit-switched. The telephone company reserves a specific physical path to the number you are calling for the duration of your call. During that time, no one else can use the physical lines involved.

Circuit-switched is often contrasted with  packet-switched. Some packet-switched networks such as the X.25 

network are able to have virtual circuit-switching. A virtual circuit-switched connection is a dedicated logicaconnection that allows sharing of the physical path among multiple virtual

Packet-switched 

Packet-switched describes the type of network in which relatively small units of data called  packets are routed throughnetwork based on the destination address contained within each packet. Breaking communication down into packetsallows the same data path to be shared among many users in the network. This type of communication betweensender and receiver is known as connectionless (rather than dedicated ). Most traffic over the Internet uses packetswitching and the Internet is basically a connectionless network.

Contrasted with packet-switched is circuit-switched, a type of network such as the regular voice telephonenetwork in which the communication circuit (path) for the call is set up and dedicated to the participants inthat call. For the duration of the connection, all resources on that circuit are unavailable for other users. Voiccalls using the Internet's packet-switched system are possible. Each end of the conversation is broken downinto packets that are reassembled at the other end.

Another type of digital network that uses packet-switching is the X.25 network, a widely-installed commerciwide area network protocol. Internet protocol packets can be carried on an X.25 network. The X.25 networkcan also support a virtual circuit in which a logical connection is established for two parties on a dedicated basis for some duration. A permanent virtual circuit (PVC) reserves the path on an ongoing basis and is an

alternative for corporations to a system of leased lines. A permanent virtual circuit is a dedicated logicalconnection but the actual physical resources can be shared among multiple logical connections or users.

Frame 

1) In telecommunications, a frame is data that is transmitted between network points as a unit complete withaddressing and necessary protocol control information. A frame is usually transmitted serial  bit  by bit andcontains a header field and a trailer field that "frame" the data. (Some control frames contain no data.)

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Here is a simple representation of a frame, based on the frame used in the frame relay access standard:

-----------Header-------  ----------Trailer---------  

Flag(01111110) 

Addressfield 

Information (data)field

(0-4096 bytes)

Frame checksequence 

Flag(01111110) 

In the figure above, the flag and address fields constitute the header. The frame check sequence and secondflag fields constitute the trailer. The information or data in the frame may contain another encapsulated framthat is used in a higher-level or different protocol. In fact, a frame relay frame typically carries data that has been framed by an earlier protocol program.

2) In time-division multiplexing (TDM), a frame is a complete cycle of events within the time division perio

3) In film and video recording and playback, a frame is a single image in a sequence of images that arerecorded and played back.

4) In computer video display technology, a frame is the image that is sent to the display image renderingdevices. It is continuously updated or refreshed from a frame buffer , a highly accessible part of video RAM

5) In artificial intelligence (AI) applications, a frame is a set of data with information about a particular object, process, or image. An example is the iris-print visual recognition system used to identify users of certain bank automated teller machines. This system compares the frame of data for a potential user with theframes in its database of authorized users.

Virtual circuit 

A virtual circuit is a circuit or path between points in a network that appears to be a discrete, physical path but isactually a managed pool of circuit resources from which specific circuits are allocated as needed to meet trafficrequirements.

A permanent virtual circuit (PVC) is a virtual circuit that is permanently available to the user just as though iwere a dedicated or leased line continuously reserved for that user. A switched virtual circuit (SVC) is a virtualcircuit in which a connection session is set up for a user only for the duration of a connection. PVCs are an

important feature of frame relay networks and SVCs are proposed for later inclusion.

Local area network 

A local area network (LAN) is a group of computers and associated devices that share a common communications linor wireless link and typically share the resources of a single processor or server within a small geographic area (for example, within an office building). Usually, the server has applications and data storage that are shared in common bmultiple computer users. A local area network may serve as few as two or three users (for example, in a home networor as many as thousands of users (for example, in an FDDI network).

Major local area network technologies are:

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 Ethernet

 Token Ring

 FDDI

 

Ethernet is by far the most commonly used LAN technology. A number of corporations use the Token Ringtechnology. FDDI is sometimes used as a  backbone LAN interconnecting Ethernet or Token Ring LANs.Another LAN technology, ARCNET, once the most commonly installed LAN technology, is still used in theindustrial automation industry.

Typically, a suite of application programs can be kept on the LAN server. Users who need an applicationfrequently can download it once and then run it from their local hard disk. Users can order printing and otheservices as needed through applications run on the LAN server. A user can share files with others at the LANserver; read and write access is maintained by a LAN administrator. A LAN server may also be used as aWeb server if safeguards are taken to secure internal applications and data from outside access.

In some situations, a wireless LAN may be preferable to a wired LAN because it is cheaper to install andmaintain.

Wide area network 

A wide area network (WAN) is a geographically dispersed telecommunications network . The term distinguishes abroader telecommunication structure from a local area network (LAN). A wide area network may be privately owned orented, but the term usually connotes the inclusion of public (shared user) networks. An intermediate form of network terms of geography is a metropolitan area network (MAN). 

Duplex 

In telecommunication, duplex communication means that both ends of the communication can send and receive signaat the same time. full-duplex communication is the same thing. half-duplex is also bidirectional communication butsignals can only flow in one direction at a time. Simplex communication means that communication can only flow inone direction and never flow back the other way.

An ordinary telephone conversation is a duplex communication. Most inexpensive speakerphones inconference rooms are half-duplex communication. (If you're speaking, you can't hear anyone else interrupt.You have to pause to let others speak.)

Gateway 

A gateway is a network  point that acts as an entrance to another network. On the Internet, a node or stopping point canbe either a gateway node or a host (end-point) node. Both the computers of Internet users and the computers thatserve pages to users are host nodes. The computers that control traffic within your company's network or at your locaInternet service provider (ISP) are gateway nodes.

In the network for an enterprise, a computer server acting as a gateway node is often also acting as a  proxy serve

and a firewall server. A gateway is often associated with both a router, which knows where to direct a given

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 packet of data that arrives at the gateway, and a switch, which furnishes the actual path in and out of thegateway for a given packet.

Runt 

In networks, a runt is a packet that is too small. For example, the Ethernet protocol requires that each packet be at leas64 bytes long. In Ethernet, which operates on the idea that two parties can attempt to get use of the line at the sametime and sometimes do, runts are usually the fragments of packet collisions. Runts can also be the result of bad wirin

or electrical interference. Runts are recorded by programs that use the Remote Network Monitoring (RNM) standardinformation base for network adminstration. RMON calls them "undersize packets".

A giant is a packet that's oversize.

*********************************************************************

Networking Basics - Key Concepts inComputer Networking

Begin your study of computer networking basics by exploring these key concepts and essential technologies

What is (Computer) Networking? Networking is the practice of linking computing devices together with hardware and software that supports

data communications across these devices.

 Networking Basics Interactive Q&AAnswer this series of common questions about basic computer networking concepts to quickly expand yourknowledge of the topic.

Visual Networking BasicsThis guide presents the essential concepts of computer networks in a sequence of visual illustrations designeto teach networking basics by example.

Connecting Two Computers for File SharingThe simplest kind of home network contains exactly two computers. You can use this kind of network toshare files, a printer or another peripheral device, and even an Internet connection. To connect two computerfor sharing network resources, consider these alternatives.

 Network RoutersA router is a small hardware device that joins multiple networks together. These networks can include wiredor wireless home networks, and the Internet.

LAN - Local Area Network A LAN supplies networking capability to a group of computers in close proximity to each other such as in aoffice building, a school or a home. LANs are useful for sharing resources or applications like files, printersand games.

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What Is a Network Protocol?Protocols serve as a language of communication among network devices. Network protocols like HTTP,TCP/IP, and SMTP provide a foundation that much of the Internet is built on. Find out more about these protocols and how they work.

TCP/IP - Transmission Control Protocol / Internet ProtocolTCP/IP provides connection-oriented communication between network devices. TCP/IP is very commonlyused both on the Internet and in home computer networks.FirewallA firewall guards a computer against unauthorized network access. Firewalls are one of the essentialelements of a safe home or business network.

 Networking Basics: EthernetEthernet is a physical and data link layer technology for local area networks (LANs). Ethernet is reliable andinexpensive, the leading standard worldwide for building wired LANs.

 Networking Basics: SwitchA network switch is a small hardware device that joins multiple computers together at a low-level network  protocol layer. Switches differ in important ways from both routers and hubs.

 Network Addresses Network addresses give computers unique identities they can use to communicate with each other.Specifically, IP addresses and MAC addresses are used on most home and business networks.

A.

In the world of computers, networking is the practice of linking two or more computing devices together fothe purpose of sharing data. Networks are built with a mix of computer hardware and computer software.

Area Networks 

 Networks can be categorized in several different ways. One approach defines the type of network accordingto the geographic area it spans. Local area networks (LANs), for example, typically reach across a singlehome, whereas wide area networks (WANs), reach across cities, states, or even across the world. The Internis the world's largest public WAN.

Network Design 

Computer networks also differ in their design.

The two types of high-level network design are called client-server  and  peer-to-peer . Client-server networksfeature centralized server computers that store email, Web pages, files and or applications. On a peer-to-peenetwork, conversely, all computers tend to support the same functions. Client-server networks are much mocommon in business and peer-to-peer networks much more common in homes.

A network topology represents its layout or structure from the point of view of data flow. In so-called "bus"networks, for example, all of the computers share and communicate across one common conduit, whereas ina "star" network, all data flows through one centralized device. Common types of network topologies includ

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 bus, ring and mesh.

Network Protocols 

In networking, the communication language used by computer devices is called the  protocol. Yet another wato classify computer networks is by the set of protocols they support. Networks often multiple protocols to

support specific applications. Popular protocols include TCP/IP, the most common protocol found on theInternet and in home networks.

Wired vs Wireless Networking 

Many of the same network protocols, like TCP/IP, work in both wired and wireless networks. Networks witEthernet cables has predominated in businesses, schools, and homes for several decades. Recently, howeverwireless networking alternatives have emerged as the premier technology for building new computer networks.

Definition: Routers are physical devices that join multiple wired or wireless networks together. Technicallya wired or wireless router is a Layer 3 gateway, meaning that the wired/wireless router connects networks (agateways do), and that the router operates at the network layer of the OSI model.

Home networkers often use an Internet Protocol (IP) wired or wireless router, IP being the most common OSnetwork layer protocol. An IP router such as a DSL or cable modem broadband router joins the home's localarea network (LAN) to the wide-area network (WAN) of the Internet.

By maintaining configuration information in a piece of storage called the "routing table," wired or wirelessrouters also have the ability to filter traffic, either incoming or outgoing, based on the IP addresses of senderand receivers.

Some routers allow the home networker to update the routing table from a Web browser interface. Broadbanrouters combine the functions of a router with those of a network switch and a firewall in a single unit.

Definition: A local area network (LAN) supplies networking capability to a group of computers in close proximity to each other such as in an office building, a school, or a home. A LAN is useful for sharingresources like files, printers, games or other applications. A LAN in turn often connects to other LANs, andto the Internet or other WAN.

Most local area networks are built with relatively inexpensive hardware such as Ethernet cables, network adapters, and hubs. Wireless LAN and other more advanced LAN hardware options also exist.

Specialized operating system software may be used to configure a local area network. For example, mostflavors of Microsoft Windows provide a software package called Internet Connection Sharing (ICS) thatsupports controlled access to LAN resources.

The term LAN party refers to a multiplayer gaming event where participants bring their own computers and

 build a temporary LAN.

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LANs, WANs, and Other Area NetworksIt’s about more than distance 

Computer networks come in many different shapes and sizes. Over the years, the networking industryhas coined terms like "LAN" and "WAN" attempting to define sensible categories for the major types of

network designs. The precise meaning of this terminology remains lost on the average person, howeve

 Area Networks

For historical reasons, the industry refers to nearly every type of network as an "area network." The mostcommonly-discussed categories of computer networks include the following -

• Local Area Network (LAN)• Wide Area Network (WAN)• Metropolitan Area Network (MAN)• Storage Area Network (SAN)• System Area Network (SAN)

• Server Area Network (SAN)• Small Area Network (SAN)• Personal Area Network (PAN)• Desk Area Network (DAN)

• Controller Area Network (CAN)• Cluster Area Network (CAN)

LANs and WANs were the original flavors of network design. The concept of "area" made good sense at thitime, because a key distinction between a LAN and a WAN involves the physical distance that the network spans. A third category, the MAN, also fit into this scheme as it too is centered on a distance-based concept.

As technology improved, new types of networks appeared on the scene. These, too, became known as variotypes of "area networks" for consistency's sake, although distance no longer proved a useful differentiator.

LAN Basics

A LAN connects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs, andoccasionally a LAN will span a group of nearby buildings. In IP networking, one can conceive of a LAN as

single IP subnet (though this is not necessarily true in practice).

Besides operating in a limited space, LANs include several other distinctive features. LANs are typicallyowned, controlled, and managed by a single person or organization. They also use certain specificconnectivity technologies, primarily Ethernet and Token Ring.

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WAN Basics

As the term implies, a wide-area network spans a large physical distance. A WAN like the Internet spans moof the world!

A WAN is a geographically-dispered collection of LANs. A network device called a router connects LANs ta WAN. In IP networking, the router maintains both a LAN address and a WAN address.

WANs differ from LANs in several important ways. Like the Internet, most WANs are not owned by any onorganization but rather exist under collective or distributed ownership and management. WANs usetechnology like ATM, Frame Relay and X.25 for connectivity.

LANs and WANs at Home

Home networkers with cable modem or DSL service already have encountered LANs and WANs in practicethough they may not have noticed. A cable/DSL router like those in the Linksys family  join the home LAN the WAN link maintained by one's ISP. The ISP provides a WAN IP address used by the router, and all of thcomputers on the home network use private LAN addresses. On a home network, like many LANs, allcomputers can communicate directly with each other, but they must go through a central gateway location toreach devices outside of their local area.

What About MAN, SAN, PAN, DAN, and CAN?

Future articles will describe the many other types of area networks in more detail. After LANs and WANs,

one will most commonly encounter the following three network designs:

A Metropolitan Area Network connects an area larger than a LAN but smaller than a WAN, such as a citywith dedicated or high-performance hardware. [1]

A Storage Area Network connects servers to data storage devices through a technology like Fibre Channel[2]

A System Area Network connects high-performance computers with high-speed connections in a cluster  configuration.

Conclusion

To the uninitiated, LANs, WANs, and the other area network acroymns appear to be just more alphabet soupin a technology industry already drowning in terminology. The names of these networks are not nearly asimportant as the technologies used to construct them, however. A person can use the categorizations as alearning tool to better understand concepts like subnets, gateways, and routers.

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Network Topologies

Bus, ring, star, and all the rest 

In networking, the term topology refers to the layout of connected devices on a network. This article

introduces the standard topologies of computer networking.

Topology in Network Design 

One can think of a topology as a network's "shape" ... (see below) ... . This shape does not necessarily correspond to the actual physical layout of the devices on the network.For example, the computers on a home LAN may be arranged in a circle, but it would be highly unlikely tofind an actual ring topology there.

 Network topologies are categorized into the following basic types:

•  bus• ring• star •

tree• mesh

More complex networks can be built as hybrids of two or more of the above basic topologies.

Bus

Bus networks (not to be confused with the system bus of a computer) use a commonbackbone to connect all devices. A single cable, the backbone functions as a sharedcommunication medium, that devices attach or tap into with an interface connector . Adevice wanting to communicate with another device on the network sends a broadcast 

message onto the wire that all other devices see, but only the intended recipient actuallyaccepts and processes the message.

Ethernet bus topologies are relatively easy to install and don't require much cabling compared to thealternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cabling optionsyears ago. However, bus networks work best with a limited number of devices. If more than a few dozen

computers are added to a bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.

Ring

In a ring network, every device has exactly twoneighbors for communication purposes. Allmessages travel through a ring in the same directio(effectively either "clockwise" or 

Bus Topology 

Ring Topology 

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"counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.

To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology. Rings are foundin some office buildings or school campuses.

Star 

Many home networks use the star topology. A star network features a central connection point called a "hubthat may be an actual hub or a switch. Devices typically connect to the hub with Unshielded Twisted Pair (UTP) Ethernet.

Compared to the bus topology, a star network generallyrequires more cable, but a failure in any star network cable will only take down one computer's network accessand not the entire LAN. (If the hub fails, however, theentire network also fails.)

Tree

Tree topologies integrate multiple star topologies together onto a bus. In its simplest form, only hub devices connectdirectly to the tree bus, and each hub functions as the"root" of a tree of devices. This bus/star hybrid approachsupports future expandability of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the

number of hub ports) alone.

Mesh

Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on amesh network can take any of several possible paths from source to destination. (Recall that in a ring,although two cable paths exist, messages can only travel in one direction.) Some WANs, like the Internet,employ mesh routing.

Conclusion

Topologies remain an important part of network design theory. You can probably build a home or small business network without understanding the difference between a bus design and a star design, butunderstanding the concepts behind these gives you a deeper understanding of important elements like hubs, broadcasts, ports, and routes.

Star Topology 

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TCP/IP Network Concepts 

As the Internet continues to grow, the use of Transmission Control Protocol/Internet Protocol (TCP/IP)

on internal networks increases. TCP/IP offers a widely deployed set of open protocol standards that arewell suited to forming wide area networks (WANs).

 

For each computer to identify itself on a TCP/IP network, each host must be uniquely identified by thefollowing information:

• An IP address A 32-bit field composed of four octets (8-bit numbers from 0 through 255).

Each address has decimal notation to separate octet numbers, such as 10.10.100.201.

• A subnet mask This indicates how the IP address is to be read. It also indicates how to

separate the network identifying information from the host identifying information and interpret

the IP address correctly. For example, if a subnet mask of 255.255.255.0 is applied for an

address 10.85.189.24, it indicates that the host number is 24, located on the 10.85.189subnetwork.

• A default gateway This is used to specify the address for the nearest routing device that is

used by the host device to forward addressed packets on to the network.

One way to configure this address information is to have a network administrator or technician

manually enter these settings on each computer before it is joined to the local network. This type of manually applied configuration is referred to as static IP addressing. It requires that all addresses are

recorded and centrally maintained by a network administrator. This provides an orderly procedure for

adding new devices and avoids problems that can arise from duplicating address assignments on thenetwork. 

For small networks, adding new devices does not present a difficult problem. However, as networksbecome larger or further divide into multiple subnetworks, integrating new devices into existing

networks becomes a significant chore for network administrators. In addition, this task may need to berepeated each time a device is moved to a different subnetwork.

Although IP addressing is an efficient scheme that works optimally for computers and routers, networkusers find common names for hosts (such as myserver.mycompany.com) much easier to remember

than IP addresses (such as 10.100.206.195). Further solutions for mapping user-friendly names toassigned IP addresses simplify user access, but compound the existing administrative problems alread

inherent in TCP/IP. 

The following sections cover some of the accepted and proposed standards that have been drafted to

provide solutions for all of these TCP/IP shortcomings.

What Is DNS?  

The Domain Name System (DNS) service was developed by the Internet Engineering Task Force (IETF)as an attempt to standardize a way to map pairings of logical host names to actual IP addresses on a

TCP/IP network.

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DNS is an acceptable method for resolving names to addresses, but it requires the use of host files, an

DNS servers must be overseen and updated manually as names and addresses are modified. This addsto tasks that a TCP/IP network administrator must perform, although it does simplify network access f

users. 

For DNS to work properly it requires the use of a name space. The DNS name space is hierarchical in

nature and allows host names to be stated in absolute or relative terms. Absolute names, or fullyqualified domain names (FQDNs) are defined from the root of the name space and uniquely identify a

node in the hierarchy. FQDNs end with a trailing dot. Relative names are represented relative to a namin the hierarchy. For example, server.microsoft.com is a relative DNS name of a host defined from the

com part of the DNS name space hierarchy, which contains domain names of commercial organizations

If the domain name of a company is: 

mycompany.com 

then the following domain names: 

sales.mycompany.com 

marketing.mycompany.com  

are known as subdomains of the mycompany.com domain. 

In addition to planning for domain hierarchies, the DNS service uses host files, which are flat text files

that pair DNS domain names and IP address pairings for hosts on the local network. Host files are useby the DNS server to process and resolve name query requests forwarded by DNS clients initiating

communications on the local network. In nearly all cases where DNS is used, host files must be createand updated manually for the DNS service to work properly for all hosts. 

Because DNS is a supported standard for all TCP/IP networks, most internal networks that supportTCP/IP have no restrictions regarding the use of DNS services. Although other TCP/IP servers that are

not Windows-based can be made to function as DNS servers, Windows NT Server provides the option oacting as a DNS server as well.

What Is WINS?  

Windows Internet Naming Service (WINS) is another option for resolving host names on networks that

use Microsoft TCP/IP. It provides a distributed database for registering and querying dynamic computename-to-IP address mappings in a routed network environment. WINS uses the NetBIOS computer

name for any Windows-based network client to register each client in the WINS database on acomputer running Windows NT Server. 

A WINS system consists of the following: 

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• WINS server Runs on a Windows NT Server–based computer and handles name registration

requests from WINS clients and registers their names and IP addresses. The server also

responds to name queries from WINS clients by returning the IP address of the name beingqueried (assuming the name is registered with the WINS server).

• WINS client Registers its name with the WINS server when it joins or leaves the network. Th

client also queries the WINS server for remote name resolution.

• WINS proxy Helps resolve names on behalf of non-WINS clients. The proxy communicates

with the WINS server to resolve names (rather than maintaining its own database) and then

caches the names for a certain amount of time. The proxy serves as an intermediary betweenthe WINS server and the non-WINS client, either by communicating with the WINS server or by

supplying a name-to-IP address mapping from its cache. The presence of a WINS proxy in aWINS system is dictated solely by the presence of non-WINS clients.

If you are administering a routed internal network, WINS is the best choice for NetBIOS nameresolution, because it is designed to solve the problems that occur with name resolution in a complex

internal network. WINS goes beyond eliminating the need for an LMHOSTS file (a hosts file of NetBIOSnames–to–IP address pairs) by reducing the use of local broadcasts for name resolution and allowing

users to easily locate systems on remote networks. 

Using WINS provides the following benefits: 

• In a dynamic network environment where host IP addresses can change frequently, WINS

provides an excellent way to dynamically register these changes as they occur.

• It centralizes management of the NetBIOS names-to-IP addresses mapping database, so that

there is no need to manage multiple LMHOSTS files across several servers on a network.

• It reduces IP broadcast traffic in a NetBIOS-based internal network, while allowing the clients to

locate remote systems across local or wide area networks easily.

• With WINS, users on a Windows NT–based network can browse transparently across routers (fo

Windows NT domains that span multiple subnets). Browsing without WINS is complicated and

involves manual procedures.

For more information on TCP/IP networking, see the Networking Supplement  in your Windows NT

Server documentation. 

What Is DHCP?  

The Dynamic Host Configuration Protocol (DHCP) was established to relieve the administrative burdenof manual configuration of TCP/IP hosts on the network. DHCP provides an alternative to static IP

addressing by enabling automatic configuration of the TCP/IP-related parameters when a DHCP client

becomes active on the local network. 

Note  DHCP provides a way of passing configuration information to hosts that is based on an earlier

protocol standard, known as the Bootstrap Protocol (BOOTP). BOOTP was conceived as a way to allowdiskless workstations to obtain TCP/IP configuration settings and perform a network "boot." DHCP uses

some of the same concepts as BOOTP, such as a shared message format, which allows existing BOOTPclients to interoperate with DHCP servers. However, further additions to DHCP distinguish it from

BOOTP. 

DHCP provides a safe, reliable, and simple TCP/IP network configuration. It ensures that address

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conflicts do not occur and helps conserve the use of IP addresses by using centralized management of 

address allocation. DHCP conserves addresses that are not in use by having the DHCP server reclaim aaddress after the address lease period  has elapsed.

The administrator controls how IP addresses are dynamically assigned by the DHCP server. To establishDHCP service, you set several parameters that are used to manage dynamic assignment of IP

addresses. First, a lease period for each address is set. A lease specifies how long a computer can usean assigned IP address before having to renew the lease with the DHCP server. Another parameter tha

must be set for DHCP service is a scope. A scope indicates the available range of addresses that can beused within DHCP assignment. The scope provides for an available pool of addresses for the DHCP

server to use when assigning and configuring clients. The scope exists only once for an entiresubnetwork, but it can be further defined by excluding reserved subranges of addresses from the pool

of assignable addresses allowed by the scope. 

Setting up a DHCP environment provides three major benefits: 

• It requires no additional address configuration for the workstation. Users do not need to know o

maintain their computer’s TCP/IP address configuration. This can be useful for mobile computerusers who use portable computers that are moved to different offices or subnets frequently.

• DHCP clients can be configured with additional TCP/IP parameters, such as the DNS server

address or WINS server addresses that are essential for successful name-to-IP address

resolution by a host.

• DHCP gives you better control of IP address assignment and management. For example, if the I

address for a router in a network changes and 250 client computers have to be updated with thnew address, you do not have to manually reconfigure every workstation.

DHCP is based on a client/server model. The DHCP server runs on a server, such as a computer runninWindows NT Server. The DHCP client runs on a network client such as a workstation running

Windows 95 or Windows NT Workstation.

During system startup (the initializing state), a DHCP client computer sends a DHCP "discover" messag

that is broadcast on the local network and can be forwarded to all the DHCP servers on the internalnetwork. Each DHCP server that receives the discover message responds with an "offer" message

containing an IP address and valid configuration information for the DHCP client computer that sent threquest. 

The DHCP client then collects the configuration and enters a selecting state, where it chooses one of thaddress "offers" provided by the DHCP servers. The client selects one of the offered configurations and

enters a requesting state. It then sends a "request" message to specify a request from that DHCP

server. The selected DHCP server sends an "acknowledgment" message that contains the address

requested, a valid lease for the address, and TCP/IP network configuration parameters for the client.The client enters a bound state after receiving the acknowledgment from the server and can nowparticipate on the TCP/IP network. 

Client computers with local disk storage save the received address for use during subsequent systemstartup. As the lease approaches its expiration date, the client attempts to renew its lease with the

DHCP server. If the DHCP server that initially offered the lease is not available or the lease has expired

the client repeats the process and gets a new configuration from any of the available DHCP servers.

The following three types of hosts can be active in a DHCP configuration process: 

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• DHCP server Contains IP addresses, lease duration, and associated TCP/IP configuration

information. The DHCP server listens for client requests and processes them.

• DHCP client Gets its IP address and related TCP/IP configuration parameters from the DHCP

server during the initialization stage. The client also extends the lease on the IP address byrenewing the lease before it expires.

• DHCP relay agent A relay agent software (compliant with RFC 1542) assists in forwarding th

DHCP packets between subnets in a routed internal network. Usually, this component runs onthe routers.

For more information about setting up DHCP services, see your Windows NT Server documentation. 

What Are Ports?  

Ports are used in TCP/IP to name the ends of logical connections that carry long-term conversations. Aport gives transport protocols such as User Datagram Protocol (UDP) and Transmission Control Protoco

(TCP) the capability of handling communications between multiple hosts. It allows communication to buniquely identified. The WinSock Proxy service uses ports extensively to provide a way of redirecting

applications. 

For the purpose of providing services to unknown callers, a service contact port is defined. Each

WinSock Proxy–enabled application specifies a port to be used as the contact port on the server for TCor UDP. The contact port is sometimes called the "well-known port." To some extent, these same port

assignments are used with UDP. To use UDP, the application must supply the IP address and portnumber of the destination application.

Ports are identified by a positive integer. Originally, the assigned ports available were in the range from0 through 255. Currently, the range for assigned ports has been expanded to allow 4-digit port numbe

up to 9,999. The assigned ports use a small portion of the range of possible port numbers, allowing

other unassigned port numbers to be designated as alternatives if the initial port is not available or is tbe used for new custom server applications. 

Ports can be designated to use either TCP or UDP as the transport-level protocol that specifies how

ports send and receive data. In addition, port assignments are enabled separately for inbound ports anoutbound ports on Microsoft Proxy Server. Inbound ports are used to listen for client requests from

Internet clients, and outbound ports are used to listen for requests from clients on the internal networ