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Networking Basics

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Internet infrastructure

Every computer that is connected to the Internet is part of a network, even the one in your 

home. For example, you may use a modem and dial a local number to connect to an

Internet Service Provider (ISP). At work, you may be part of a local area network 

(LAN), but you most likely still connect to the Internet using an ISP that your company

has contracted with. When you connect to your ISP, you become part of their network.The ISP may then connect to a larger network and become part of their network. The

Internet is simply a network of networks.

Most large communications companies have their own dedicated backbones connecting

various regions. In each region, the company has a Point of Presence (POP). The POP isa place for local users to access the company's network, often through a local phone

number or dedicated line. The amazing thing here is that there is no overall controlling

network. Instead, there are several high-level networks connecting to each other through

Network Access Points or NAPs.

When you connect to the Internet, your computer becomes part of a network.

Internet Network Example

Here's an example. Imagine that Company A is a large ISP. In each major city, Company

A has a POP. The POP in each city is a rack full of modems that the ISP's customers dial

into. Company A leases fiber optic lines from the phone company to connect the POPs

together (see, for example, this UUNET Data Center Connectivity Map).

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Imagine that Company B is a corporate ISP. Company B builds large buildings in major 

cities and corporations locate their Internet server machines in these buildings. Company

B is such a large company that it runs its own fiber optic lines between its buildings sothat they are all interconnected.

In this arrangement, all of Company A's customers can talk to each other, and all of Company B's customers can talk to each other, but there is no way for Company A's

customers and Company B's customers to intercommunicate. Therefore, Company A andCompany B both agree to connect to NAPs in various cities, and traffic between the two

companies flows between the networks at the NAPs.

In the real Internet, dozens of large Internet providers interconnect at NAPs in various

cities, and trillions of bytes of data flow between the individual networks at these points.The Internet is a collection of huge corporate networks that agree to all intercommunicate

with each other at the NAPs. In this way, every computer on the Internet connects to

every other.

The Function of an Internet Router All of these networks rely on NAPs, backbones and routers to talk to each other. What is

incredible about this process is that a message can leave one computer and travel halfwayacross the world through several different networks and arrive at another computer in a

fraction of a second!

The routers determine where to send information from one computer to another. Routers

are specialized computers that send your messages and those of every other Internet user speeding to their destinations along thousands of pathways. A router has two separate,

 but related, jobs:

• It ensures that information doesn't go where it's not needed. This is crucial for 

keeping large volumes of data from clogging the connections of "innocent bystanders."

• It makes sure that information does make it to the intended destination.

In performing these two jobs, a router is extremely useful in dealing with two separatecomputer networks. It joins the two networks, passing information from one to the other.

It also protects the networks from one another, preventing the traffic on one from

unnecessarily spilling over to the other. Regardless of how many networks are attached,

the basic operation and function of the router remains the same. Since the Internet is one

huge network made up of tens of thousands of smaller networks, its use of routers is anabsolute necessity.

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Internet Backbone

The National Science Foundation (NSF) created the first high-speed backbone in 1987.Called NSFNET, it was a T1 line that connected 170 smaller networks together and

operated at 1.544 Mbps (million bits  per second). IBM, MCI and Merit worked with NSF

to create the backbone and developed a T3 (45 Mbps) backbone the following year.

Backbones are typically fiber optic trunk lines. The trunk line has multiple fiber opticcables combined together to increase the capacity. Fiber optic cables are designated OC

for optical carrier, such as OC-3, OC-12 or OC-48. An OC-3 line is capable of 

transmitting 155 Mbps while an OC-48 can transmit 2,488 Mbps (2.488 Gbps). Comparethat to a typical 56K modem transmitting 56,000 bps and you see just how fast a modern

 backbone is.

Today there are many companies that operate their own high-capacity backbones, and all

of them interconnect at various NAPs around the world. In this way, everyone on the

Internet, no matter where they are and what company they use, is able to talk to everyoneelse on the planet. The entire Internet is a gigantic, sprawling agreement between

companies to intercommunicate freely.

Internet Protocol: IP Addresses

Every machine on the Internet has a unique identifying number, called an IP Address.

The IP stands for Internet Protocol, which is the language that computers use to

communicate over the Internet. A protocol is the pre-defined way that someone who

wants to use a service talks with that service. The "someone" could be a person, but moreoften it is a computer program like a Web browser.

A typical IP address looks like this:

216.27.61.137

To make it easier for us humans to remember, IP addresses are normally expressed in

decimal format as a dotted decimal number like the one above. But computerscommunicate in binary form. Look at the same IP address in binary:

11011000.00011011.00111101.10001001

The four numbers in an IP address are called octets, because they each have eight positions when viewed in binary form. If you add all the positions together, you get 32,which is why IP addresses are considered 32-bit numbers. Since each of the eight

 positions can have two different states (1 or zero), the total number of possible

combinations per octet is 28 or 256. So each octet can contain any value between zero and255. Combine the four octets and you get 232 or a possible 4,294,967,296 unique values!

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Out of the almost 4.3 billion possible combinations, certain values are restricted from use

as typical IP addresses. For example, the IP address 0.0.0.0 is reserved for the default

network and the address 255.255.255.255 is used for  broadcasts.

The octets serve a purpose other than simply separating the numbers. They are used to

create classes of IP addresses that can be assigned to a particular business, government or other entity based on size and need. The octets are split into two sections: Net and Host.

The Net section always contains the first octet. It is used to identify the network that acomputer belongs to. Host (sometimes referred to as Node) identifies the actual computer 

on the network. The Host section always contains the last octet. There are five IP classes

 plus certain special addresses.

Internet Protocol: Domain Name System

When the Internet was in its infancy, it consisted of a small number of computers hooked

together with modems and telephone lines. You could only make connections by

 providing the IP address of the computer you wanted to establish a link with. For example, a typical IP address might be 216.27.22.162. This was fine when there were

only a few hosts out there, but it became unwieldy as more and more systems came

online.

The first solution to the problem was a simple text file maintained by the Network 

Information Center that mapped names to IP addresses. Soon this text file became so

large it was too cumbersome to manage. In 1983, the University of Wisconsin created the

Domain Name System (DNS), which maps text names to IP addresses automatically.

This way you only need to remember www.howstuffworks.com, for example, instead of 

HowStuffWorks.com's IP address.

URL: Uniform Resource Locator 

When you use the Web or send an e-mail message, you use a domain name to do it. For 

example, the Uniform Resource Locator (URL) "http://www.howstuffworks.com"

contains the domain name howstuffworks.com. So does this e-mail address:

example@howstuffworks.com. Every time you use a domain name, you use the Internet'sDNS servers to translate the human-readable domain name into the machine-readable IP

address. Check out How Domain Name Servers Work for more in-depth information on

DNS.

Top-level domain names, also called first-level domain names, include .COM, .ORG,.NET, .EDU and .GOV. Within every top-level domain there is a huge list of second-

level domains. For example, in the .COM first-level domain there is:

• HowStuffWorks

• Yahoo

• Microsoft

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Every name in the .COM top-level domain must be unique. The left-most word, like

www, is the host name. It specifies the name of a specific machine (with a specific IP

address) in a domain. A given domain can, potentially, contain millions of host names aslong as they are all unique within that domain.

DNS servers accept requests from programs and other name servers to convert domainnames into IP addresses. When a request comes in, the DNS server can do one of four 

things with it:

1. It can answer the request with an IP address because it already knows the IP

address for the requested domain.

2. It can contact another DNS server and try to find the IP address for the name

requested. It may have to do this multiple times.3. It can say, "I don't know the IP address for the domain you requested, but here's

the IP address for a DNS server that knows more than I do."

4. It can return an error message because the requested domain name is invalid or 

does not exist.

A DNS Example

Let's say that you type the URL www.howstuffworks.com into your browser. The browser contacts a DNS server to get the IP address. A DNS server would start its search

for an IP address by contacting one of the root DNS servers. The root servers know the

IP addresses for all of the DNS servers that handle the top-level domains (.COM,.NET, .ORG, etc.). Your DNS server would ask the root for www.howstuffworks.com,

and the root would say, "I don't know the IP address for www.howstuffworks.com, but

here's the IP address for the .COM DNS server."

Your name server then sends a query to the .COM DNS server asking it if it knows the IPaddress for www.howstuffworks.com. The DNS server for the COM domain knows the

IP addresses for the name servers handling the www.howstuffworks.com domain, so it

returns those.

Your name server then contacts the DNS server for www.howstuffworks.com and asks if it knows the IP address for www.howstuffworks.com. It actually does, so it returns the IP

address to your DNS server, which returns it to the browser, which can then contact the

server for www.howstuffworks.com to get a Web page.

One of the keys to making this work is redundancy. There are multiple DNS servers atevery level, so that if one fails, there are others to handle the requests. The other key is

caching. Once a DNS server resolves a request, it caches the IP address it receives. Once

it has made a request to a root DNS server for any .COM domain, it knows the IP addressfor a DNS server handling the .COM domain, so it doesn't have to bug the root DNS

servers again for that information. DNS servers can do this for every request, and this

caching helps to keep things from bogging down.

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Even though it is totally invisible, DNS servers handle billions of requests every day and

they are essential to the Internet's smooth functioning. The fact that this distributed

database works so well and so invisibly day in and day out is a testimony to the design.

Internet Servers and Clients

Internet servers make the Internet possible. All of the machines on the Internet are either 

servers or clients. The machines that provide services to other machines are servers. Andthe machines that are used to connect to those services are clients. There are Web servers,

e-mail servers, FTP servers and so on serving the needs of Internet users all over the

world.

When you connect to www.howstuffworks.com to read a page, you are a user sitting at aclient's machine. You are accessing the HowStuffWorks Web server. The server machine

finds the page you requested and sends it to you. Clients that come to a server machine do

so with a specific intent, so clients direct their requests to a specific software server 

running on the server machine. For example, if you are running a Web browser on your machine, it will want to talk to the Web server on the server machine, not the e-mail 

server.

A server has a static IP address that does not change very often. A home machine that isdialing up through a modem, on the other hand, typically has an IP address assigned by

the ISP every time you dial in. That IP address is unique for your session -- it may be

different the next time you dial in. This way, an ISP only needs one IP address for eachmodem it supports, rather than one for each customer.

Ports and HTTP

Any server machine makes its services available using numbered ports -- one for each

service that is available on the server. For example, if a server machine is running a Webserver and a file transfer protocol (FTP) server, the Web server would typically be

available on port 80, and the FTP server would be available on port 21. Clients connect to

a service at a specific IP address and on a specific port number.

Once a client has connected to a service on a particular port, it accesses the service usinga specific protocol. Protocols are often text and simply describe how the client and server 

will have their conversation. Every Web server on the Internet conforms to the hypertext

transfer protocol (HTTP). You can learn more about Internet servers, ports and

 protocols by reading How Web Servers Work .

 Networks, routers, NAPs, ISPs, DNS and powerful servers all make the Internet possible.

It is truly amazing when you realize that all this information is sent around the world in a

matter of milliseconds! The components are extremely important in modern life --without them, there would be no Internet. And without the Internet, life would be very

different indeed for many of us.

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Difference between Hub,Switch and Router

What's the difference between a Hub, a Switch and a Router?

In a word: intelligence.

Hubs, switches, and routers are all devices that let you connect one or more computers toother computers, networked devices, or to other networks. Each has two or more

connectors called ports into which you plug in the cables to make the connection.

Varying degrees of magic happen inside the device, and therein lies the difference. I oftensee the terms misused so let's clarify what each one really means.

•

A hub is typically the least expensive, least intelligent, and least complicated of the three.Its job is very simple: anything that comes in one port is sent out to the others. That's it.Every computer connected to the hub "sees" everything that every other computer on the

hub sees. The hub itself is blissfully ignorant of the data being transmitted. For years,

simple hubs have been quick and easy ways to connect computers in small networks.

A switch does essentially what a hub does but more efficiently. By paying attention tothe traffic that comes across it, it can "learn" where particular addresses are. For example,

if it sees traffic from machine A coming in on port 2, it now knows that machine A is

connected to that port and that traffic to machine A needs to only be sent to that port and

not any of the others. The net result of using a switch over a hub is that most of the

network traffic only goes where it needs to rather than to every port. On busy networksthis can make the network significantly faster 

A router is the smartest and most complicated of the bunch. Routers come in all shapesand sizes from the small four-port broadband routers that are very popular right now to

the large industrial strength devices that drive the internet itself. A simple way to think of 

a router is as a computer that can be programmed to understand, possibly manipulate, and

route the data its being asked to handle. For example, broadband routers include theability to "hide" computers behind a type of firewall which involves slightly modifying

the packets of network traffic as they traverse the device. All routers include some kind

of user interface for configuring how the router will treat traffic. The really large routers

include the equivalent of a full-blown programming language to describe how theyshould operate as well as the ability to communicate with other routers to describe or 

determine the best way to get network traffic from point A to point B.

A quick note on one other thing that you'll often see mentioned with these devices andthat's network speed. Most devices now are capable of both 10mps (10 mega-bits, or 

million bits, per second) as well as 100mbs and will automatically detect the speed. If the

device is labeled with only one speed then it will only be able to communicate with

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devices that also support that speed. 1000mbs or "gigabit" devices are starting to slowly

 become more common as well. Similarly many devices now also include 802.11b or 

802.11g wireless transmitters that simply act like additional ports to the device

NAP(Network Access Point)

Short for network access point, a public network exchange facility where Internet

Service Providers (ISPs) can connect with one another in peering 

arrangements. The NAPs are a key component of the Internet backbone 

 because the connections within them determine how traffic is routed. They are

also the points of most Internet congestion. Explain Transmission Control

Protocol, TCP

TCP ensures reliable and end to end delivery of segments of information. Segments areacknowledged to the source when received by the destination. Data is broken up into

segments and sequenced properly before transmission. This arrangement of segments

allows destination to trace lost data in transmission.

What is TCP protocol?

Transmission control Protocol is used to establish communication between nodes or networks and exchange data packets. It guarantees delivery of data packets in the order 

they were sent. Hence it is most commonly used in all applications that require

guaranteed delivery of data. It can handle both timeouts (if packets were delayed) andretransmission (if packets were lost). The stream of data is transmitted in segments. The

segment header is 32 bit. it is a connectionless communication protocol at the third level

(network) of the OSI model.

TCP vs. UDP.

TCP guarantees the delivery of data. UDP on the other hand, does not guarantee delivery

of data. TCP delivers messages in the order they were sent. UDP has no ordering

mechanisms. In TCP data is sent as a stream while UDP sends data as individual packets.

UDP is faster than TCP. TCP is a connection oriented protocol while UDP isconnectionless.

What is TCP windowing concept?

TCP windowing concept is primarily used to avoid congestion in the traffic. It controlsthe amount of unacknowledged data a sender can send before it gets an acknowledgement back from the receiver that it has received it.

What is Trusted and Untrusted Networks?

Trusted networks: Such Networks allow data to be transferred transparently. The

machines using a trusted network are usually administered by an Administrator to ensure

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that private and secured data is not leaked. Access to this network is limited. Computers

using trusted networks are more secured and confidential because of strong firewalls.

Untrusted networks: Such networks are usually administered by the owners. They canallow improper access to sensitive or personal data. These machines are usually separate.

Such machines could me more prone to attacks.

 Network load balancing

 N etwork  Load  Balancing (NLB) is a clustering technology offered by Microsoft as part

of all Windows 2000 Server and Windows Server 2003 family operating

systems. NLB uses a distributed algorithm to load balance network  traffic 

across a number of hosts, helping to enhance the scalability and availability of 

mission critical, IP-based services, such as Web, Virtual Private Networking,streaming media, terminal services, proxy and so on. It also provides high

availability by detecting host failures and automatically redistributing traffic to

operational hosts.

Default Gateway

In a network using subnets, the router that forwards traffic to a destination outside of thesubnet of the transmitting device.

Subnet mask 

It is used to determine what subnet an IP address belongs to. An IP address has two

components, the network address and the host address. For example, consider 

the IP address 150.215.017.009. Assuming this is part of a Class B network, thefirst two numbers (150.215) represent the Class B network address, and thesecond two numbers (017.009) identify a particular host on this network.

What are the different types of VPN?

• Remote Access VPN:- Also called as Virtual Private dial-up network (VPDN) is

mainly used in scenarios where remote access to a network becomes essential.Remote access VPN allows data to be accessed between a company’s private

network and remote users through a third party service provider; Enterprise

service provider. E.g Sales team is usually present over the globe. Using Remote

access VPN, the sales updates can be made.• Site to Site VPN – Intranet based: This type of VPN can be used when multiple

Remote locations are present and can be made to join to a single network.

Machines present on these remote locations work as if they are working on asingle network.

• Site to Site VPN – Extranet based: This type of VPN can be used when several

different companies need to work in a shared environment. E.g. Distributors andservice companies. This network is more manageable and reliable.

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What are the different authentication methods used in VPNs?

The authentication method uses an authentication protocol. The methods are:

• EAP authentication method: Extensible authentication protocol authenticates

remote access connection. The authentication mechanism is decided between theremote VPN client and authenticator (ISA). The mechanism is typical in which

authenticator requests for authentication information and the responses are given

 by the remote VPN client.

• MS Chap Authentication method: Microsoft Challenge Handshake 

Authentication Protocol (MS-CHAP) starts with the authenticator (Remote access

server) challenge. The challenge to the remote access client sends a session

identifier and challenge string. The client in response sends the nonreversibleencryption of the string, the identifier and password. Authenticator checks the

credentials and grants access on a successful authentication.

• Unencrypted passwords (PAP):- Uses plain text passwords. Does not involve

encryption. Used for less secure clients.• Shiva Password Authentication Protocol (SPAP):- It is a password

authentication protocol. It is less secure as the same user password is always sentin the same reversibly encrypted form

What is Tunneling?

Tunneling is a mechanism provided to transfer data securely between two networks. The

data is split into smaller packets and passed through the tunnel. The data passing through

the tunnel has 3 layers of encryption. The data is encapsulated. Tunneling can beapproached by Point to Point tunneling protocol.

What are voluntary and compulsory tunnels?

Voluntary Tunneling

Users computer is an end point of the tunnel and acts as tunnel client. Here the client or user issues a request to configure and create a voluntary tunnel. They require a dial up or 

LAN connection. Example of dial up connection is internet at home where a call is made

to the ISP and connection is obtained.

Compulsory tunnelingIn compulsory tunneling, instead of the user a vpn remote access server configures and

creates a tunnel. Hence, the end point is the Remote sever not the user.

Explain static and dynamic tunnels.

Tunnels that are created manually are static tunnels. Tunnels that are auto discovered aredynamic tunnels. In dynamic tunneling, tcp connections can be checked dynamically. If 

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no connections exist that are routed through the tunnel, a check for more suitable gateway

can be done. Static tunneling may at times require dedicated equipments.

Explain the importance of increasing and decreasing subnet bits.

Adding 1’s as a bit means increasing the subnets and decreasing the hosts. Removing or decreasing 1’s means decreasing subnets and increasing hosts. Hence by increasing or 

decreasing the subnet architecture can be decided as per needs.

Why do we need a subnet mask?

A subnet mask allows identification of host part and network part of an IP address.Subnet mask can be used to find if an IP address is present on a subnet or not.

What is RTP?

Real-Time Transfer Protocol lays a standard or a way to transfer or manage real time dataover a network. It does not guarantee the delivery of data or provide any quality of 

service. However it helps to manage the data. Which means that RTP can be used deliver the necessary data to the application to make sure it can put the received packets in the

correct order Real time data examples could be audio and video.

What is RTP Multiplexing?

RTP multiplexing allows multiple media flows within a single RTP data payload betweentwo points. This means that it can be used to carry multiple streams of data in one RTP

 packet. RTP multiplexing will reduce the bandwidth used. RTP multiplexing will also

increase scalability.

Explain the use of RTP and RTCP protocols.

Use of RTP and RTCP:-

1. RTP can be used to transfer Real time data like voice packets.

2. RTP can be used with RTCP which makes it possible to monitor data.

3. Packet loss can be detected by RTP using Sequence number 

RTCP provides Qos feedback :- Packets lost, round trip time.

Describe the format of RTP and RTCP packets.

The 32 bits of RTP packet format is as follows:- (L to R)

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Bit 0-1:- Indicates version, currently 2

Bit 2:- P- indicates padding bytes

Bit 3:- X- Indicates presence of extension header Bit 4-7:- CC- Contains number of CSRC identifiers that follows the header 

Bit 8:- M- Current data has some special relevance (if set)

Bit 9-15:- PT- Indicates format of payloadBit 16-31:- Sequence number 

Timestamp: - 32bits – time stamp of packet

SSRC- Synchronization source identifier uniquely identifies the source of a stream.CSRC -Contributing source IDs enumerate contributing sources to a stream which has

 been generated from multiple sources

Extension header: - first 32 bit word contains profile specific identifier and length

specifier 

The 32 bits of RTCP header format is as follows:- (L to R)

Bit 0-1:- Indicates version, currently 2

Bit 2:- P- indicates padding bytesBit 3 to 7:- Count of number of reception report blocksBit 8 to 15:- Type: - Determined RTCP packet type. Type can take values from 0 to 255

16 to 31:- Length- Length of RTCP packet - 1

SR: - Sender Report for transmission and reception from active sendersRR: - Receiver report for reception from in active senders

SDES: - Source description items

BYE- indicates end of participationAPP: - Application specific functions

Describe how the multicast protocol works.

Multicast protocol or Internet protocol delivers a singles message to multiple machines.

One packet from the source is replicated and sent to the destination. Every multicast

message requires a multi case group. The group defines the addresses which will receivethe message. The group is defined by the class D address. Different routing protocols are

used to find the multicast groups and build routes for them. Distance Vector Multicast

 protocol is one of them. The receiver, to whom the multicast packet is sent to, needs to‘join’ the group. Joining the group is enabled and managed by IGMP. Multicast routers

are used to transmit the messages from one network to another.

Describe how to control the scope of multicast transmissions.

Controlling the scope of multicast transmission restricts the range of group members.TTL (Time To Live) is one of the mechanisms to limit the scope. If the TTL value is

small, packets would only be multicast to smaller distance destinations. More the value of 

TTL, more would be the scope of transmission to a larger number of machines.

Administrative scoping is another mechanism. In this, transmission is restricted to aspecific address space of an organization or a site.

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Explain why use Multicasting.

• When the same message or packet needs to be sent to multiple destinations,

multicasting is used.

• Within campus and offices, using multicasting file distribution can be done.

•

System messages, news and videos can be sent at the same time.• More commonly used for audio and video streaming.

What is a socket?

A socket is used to connect an application to a network protocol. A socket enablescommunication between a client and a server. The communication is started when the

client is assigned a local port number, and binds a socket to it. The client writes on the

socket and gets information from server by reading it.

What are Sockets? How do Sockets Work?

A socket is used to connect an application to a network protocol. A socket enables

communication between a client and a server. The communication is started when the

client is assigned a local port number, and binds a socket to it. The client writes on the

socket and gets information from server by reading it. The Socket class is used tocommunicate. It provides rich set of methods for both asynchronous and synchronous

data transfer. ConnectAsynch is used to start an asynchronous connection. SendAsynch

and ReceiveAsynch are used to send and receive data. Shutdown and close methods areused to shutdown and close the sockets.

Datagram vs. stream.

Stream can be considered as a pipe that allows full duplex connection. A datagram or a

 packet on the other hand, has a source and a destination. There is no connection. Streamis like a communication channel while datagram is completely self contained. Streams

 provide a reliable and sequenced communication. Datagram’s on the other hand are

unreliable and no sequence maintained.

What is a stream socket?

A stream socket provides two way communications between a client and server. Thiscommunication is reliable and sequenced. Stream sockets are above TCP to run across

any networks. They provide unduplicated flow of data and have well establishedmechanism for creating and destroying connections and for detecting errors.

What is SSL?

SSL is Secured Socket Layer. SSL is used to establish a secured and an encrypted

connection between a server and the browser. SSL is most commonly seen in payment

and banking web sites. To create a SSL connection, a SSL certificate needs to be created.

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If the website has a SSL certificate installed, a small icon is displayed in the tool bar to

make customers confident that the site is secured.

What are the two important TCP Socket classes? Explain them

The two most important socket Classes are:

• Socket Class: The Socket class provides enriched methods and properties for network communications. It allows both synchronous and asynchronous data

transfer. Shutdown method should be used to end the Socket once the data

transfer is complete. Using SetSocketOption the socket can be configured.Declaration:

Public Class Socket _ Implements IDisposable

• Server Socket Class: The server Socket class is used to implement server 

sockets. Server sockets are used to respond to requests received over the network.

What are the advantages and disadvantages of Java Sockets?

Advantages:

• Flexible and powerful.

• Cause low network traffic if efficiently used.

• Only updated information can be sent.

Disadvantages:

• The Java applets can establish communication only with the machine requestedand not with any other machine on the network.

• Sockets allow only raw data to be sent. This means that both client and server 

need to have mechanisms to interpret the data.

What is a Network? 

A network consists of two or more computers that are linked in order to share resources(such as printers and CDs), exchange files, or allow electronic communications. The

computers on a network may be linked through cables, telephone lines, radio waves,

satellites, or infrared light beams.

The two basic types of networks include:

• Local Area Network (LAN)

• Wide Area Network (WAN)

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Internet requests that require Network Address Translation (NAT) are quite complex but

happen so rapidly that the end user rarely knows it has occurred. A workstation inside a

network makes a request to a computer on the Internet. Routers within the network recognize that the request is not for a resource inside the network, so they send the

request to the firewall. The firewall sees the request from the computer with the internal

IP. It then makes the same request to the Internet using its own public address, andreturns the response from the Internet resource to the computer inside the private

network. From the perspective of the resource on the Internet, it is sending information to

the address of the firewall. From the perspective of the workstation, it appears thatcommunication is directly with the site on the Internet. When NAT is used in this way, all

users inside the private network access the Internet have the same public IP address when

they use the Internet. That means only one public addresses is needed for hundreds or 

even thousands of users.

Most modern firewalls are stateful - that is, they are able to set up the connection between

the internal workstation and the Internet resource. They can keep track of the details of 

the connection, like ports, packet order, and the IP addresses involved. This is calledkeeping track of the state of the connection. In this way, they are able to keep track of thesession composed of communication between the workstation and the firewall, and the

firewall with the Internet. When the session ends, the firewall discards all of the

information about the connection.

There are other uses for Network Address Translation (NAT) beyond simply allowing

workstations with internal IP addresses to access the Internet. In large networks, some

servers may act as Web servers and require access from the Internet. These servers are

assigned public IP addresses on the firewall, allowing the public to access the serversonly through that IP address. However, as an additional layer of security, the firewall acts

as the intermediary between the outside world and the protected internal network.Additional rules can be added, including which ports can be accessed at that IP address.Using NAT in this way allows network engineers to more efficiently route internal

network traffic to the same resources, and allow access to more ports, while restricting

access at the firewall. It also allows detailed logging of communications between thenetwork and the outside world.

Additionally, NAT can be used to allow selective access to the outside of the network,

too. Workstations or other computers requiring special access outside the network can be

assigned specific external IPs using NAT, allowing them to communicate with computersand applications that require a unique public IP address. Again, the firewall acts as the

intermediary, and can control the session in both directions, restricting port access and

 protocols.

 NAT is a very important aspect of firewall security. It conserves the number of publicaddresses used within an organization, and it allows for stricter control of access to

resources on both sides of the firewall.

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What is a Router?

Before we delve into the world of routers, you need to understand what a broadbandconnection is and how it works. According to Dictionary.com, broadband is "a high-

speed, high-capacity transmission medium that can carry signals from multiple

independent network carriers. This is done on a single coaxial or fiber-optic cable byestablishing different bandwidth channels. Broadband technology can support a widerange of frequencies. It is used to transmit data, voice and video over long distances

simultaneously."

Routers take information that arrives through your broadband signal via a modem,decipher it, and deliver it to your computer. The router will also choose the best route for 

the data packet so that you receive the information quickly.

Many different types of routers have been developed so that the information coming over 

your broadband connection can be sent to a variety of different receivers including your 

computer, your phone, and others.

Types of Routers

There are several types of routers that you will want to understand. You need to know the

difference so that you can set up your network or at least so that you can understand whatthe local computer guy tells you to do.

Broadband Routers

Broadband routers can be used to do several different types of things. They can be used

to connect two different computers or to connect two computers to the Internet. They canalso be used to create a phone connection.

If you are using Voice over IP (VoIP) technology, then you will need a broadband router 

to connect your Internet to your phone. These are often a special type of modem that will

have both Ethernet and phone jacks. Although this may seem a little confusing, simplyfollow the instructions that your VoIP provider sends with your broadband router -

usually you must purchase the router from the company in order to obtain the service.

Wireless Routers

Wireless routers connect to your modem and create a wireless signal in your home or office. So, any computer within range can connect to your wireless router and use your 

 broadband Internet for free. The only way to keep anyone from connecting to your 

system is to secure your router.

A word of warning about wireless routers: Be sure your secure them, or you will besusceptible to hackers and identity thieves. In order to secure your router, you simply

need to come to WhatIsMyIPAddress.com, and get your IP address. Then, you'll type that

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into your web browser and log into your router (the user ID and password will come with

your router).

Remember to change the router ID and password as well or someone else might lock youout of your system.

Once there, visit the wireless settings, type in a security key and you'll be about ready to

go. Make sure you write down the key or save it in a Word document. When you try to

log onto your Internet connection, you'll just need to enter the key. Your computer shouldremember the key, but write it down somewhere for safe keeping just in case

What is a Private IP Address?

When you send a letter from your house to a friend, you have to know the address to sendit to so that the postman knows which street and which house to take it to. Computer 

networks such as the Internet are no different except instead of sending your web traffic

to "1145 Main Street"; your computer's location is known as an IP Address.

An IP address is your computer's equivalent of your postal address and just like the mailservice, each computer has to have its own address so that it will only receive the

information that is meant for it and not anything that is meant for someone else.

While we are used to writing out streets and house numbers on envelopes, inside your 

computer IP Addresses are usually represented in what is known as dotted-decimalformat such as 124.62.112.7 as this is the system that is understood by computers. As you

can see, the address is split into 4 sections known as "octets" and each of the four octets

can be numbered from 0-255, providing a total of 4,294,967,296 potentially unique IP

Addresses.

 Now, while 4.2 Billion might seem like a lot, for many years large amounts of these have

 been allocated and used by large network such as backbone providers, ISPs and large

Universities that made up the early Internet While other groups still have been reservedfor special purposes and are not usable, so in practice the real amount is far less than 4.2

 billion. The problem that we face today is that with many homes owning more than one

computer and with cell phones, PDAs and even fridges being enabled for Internet access

these days, IP Addresses are running out.

When I mentioned above that some blocks of addresses had been reserved for special

 purposes, one of these purposes was for private networking and it is these privateaddresses that help to relieve the pressure on the remaining address space and make possible many of the cable and DSL routers that people have at home today to share their 

Internet connection amongst many PCs.

Private IP address ranges

The ranges and the amount of usable IP's are as follows:

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10.0.0.0 - 10.255.255.255

Addresses: 16,777,216

172.16.0.0 - 172.31.255.255Addresses: 1,048,576

192.168.0.0 - 192.168.255.255

Addresses: 65,536

So, what are these addresses and how do they work? 

For example, if I had 6 computers that I wanted to network, I might number them from172.16.0.1 up to 172.16.0.6 and this would still leave over a million more addresses that I

could use if I were to buy some more computers or if I was networking a large office and

needed lots and lots of addresses.

These blocks of addresses can be used by anyone, anywhere - even if your neighbor isusing the exact same addresses this won't cause a problem. This is possible because these

addresses are known as "non-Routable addresses" and the devices on the internet that

move data from one place to another are specially programmed to recognize theseaddresses. These devices (known as routers) will recognize that these are private

addresses belonging to your network and will never forward your traffic onto the Internet

so for your connection to work; you will always require at least one real address from the

general pool so that your home router can perform what is known as " Network AddressTranslation".

 NAT is a process where your router changes your private IP Address into a public one so

that it can send your traffic over the Internet, keeping track of the changes in the process.When the information comes back to your router, it reverses the change back from a realIP Address into a private one and forwards the traffic back to your computer.

Private addresses and NAT is what makes your home router work and by using them,

anyone is able to connect as many computer's as they wish to the Internet without havingto worry about running out of addresses and this gives everyone many more years until

all the available addresses are used up.

How DHCP Works

As its name indicates, DHCP provides dynamic IP address assignment. What this meansis that instead of having to rely on a specific IP address, a computer will be assigned onethat is available from a subnet or "pool" that is assigned to the network. DHCP also

extends BOOTP functionality to provide IP addresses that expire. BOOTP indirectly uses

a form of leasing that never expired, but the term wasn't actually used until theintroduction of DHCP. When DHCP assigns an IP address, it actually leases the identifier 

to the host computer for a specific amount of time. The default lease is five days, but a

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network administrator should evaluate their own particular circumstances to determine an

appropriate lease.

In basic terms, the DHCP lease process works as follows:

1. A network device attempts to connect to the Internet.2. The network requests an IP address.

3. The DHCP server allocates (leases) the network device an IP address, which is

forwarded to the network by a router.4. DHCP updates the appropriate network servers with the IP address and other 

configuration information.

5. The network device accepts the IP address.

6. The IP address lease expires.7. DHCP either reallocates the IP address or leases one that is available.

8. The network device is no longer connected to the Internet.

9. The IP address becomes an available address in the network pool of IP addresses.

To set up DHCP, you basically need a DHCP-supported client (at least one) and router,

and a DHCP server. The client is a computer or other device on a network that requires

an IP address and or other network configuration information. The router functions as a

forwarding (or routing) agent of IP address requests from the DHCP server. The DHCPserver is key to the entire operation. It is responsible for allocating, leasing, reallocating,

and renewing IP addresses. Windows and Linux both support DHCP software.

What is VOIP?

"VOIP - Short for Voice Over Internet Protocol, a category of hardware and software that

enables people to use the Internet as the transmission medium for telephone calls bysending voice data in packets using IP rather than by traditional circuit transmissions..."(quoted from Webopedia Computer Dictionary).

Uses

VOIP has become a growing method of communication in the modern world, especially

in sales-oriented jobs. It has been integrated mostly for business purposes to replaceconventional phone calls. The methods of use can be classified into three major 

categories:

• To interconnect VOIP phones within a single building using the building's LocalArea Network (LAN)

• To interconnect VOIP phones between multiple locations of the same

organization using a backbone network 

• To interconnect VOIP phones to regular telephones in different organizations or 

for sales purposes

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Benefits

Toll-Free Calls

The main purpose of VOIP is to provide a method of cheap yet extensive communication.

Regular telephone calls can become costly if used too frequently, yet with VOIP one onlyneeds to pay for a good internet connection and subscription to a VOIP service. With

these two facilities handy, VOIP becomes a capability to make limitless phone calls

through the internet. Although the person at the other end may be using a regular phone,the VOIP user either uses a computer or VOIP phone to communicate.

The major advantage of VOIP is that it allows the user to make as many phone calls as

they like for a fixed price instead of being charged by the minute. This makes a huge

cutback on costs for jobs such as telephone marketing that require making many phonecalls for hours every day.

VOIP can become low-cost or toll-free for people who are using only the internet tocommunicate. If both or all users are using the internet to communicate, VOIP becomes a

way of talking or conferencing without having to pay any additional fee apart from theregular fee for internet.

Reduced Wiring Costs

Using VOIP can also minimize building wiring costs in new or renovated buildings due

to reduced wire infrastructure. Instead of using many cable systems such as datanetworking, telephone connections, and cable distribution, VOIP makes it possible to rely

on less cabling costs in terms of maintenance as well as initial installment.

Easy Relocation

Anyone who has relocated their job or home knows how difficult it is to switch local phone numbers, not only for them but even more so for others who want to call them.

Cell phones have largely helped to solve this problem, but cell phones are one of the

costliest methods of common communication. VOIP provides a cheaper solution, sinceinternet is a universal medium. A VOIP instrument can be relocated without central

administrative support. With the internet being a global medium, VOIP allows users to

communicate from anywhere in the world at one of the lowest costs possible.

Roaming Facility

To use your regular local phone you have to access it from your regular phone line. It

would not be possible for you to make a phone call using your local phone line if you are

somewhere else using someone else's phone. Here is another problem that VOIP helps usovercome. With VOIP, you can use someone else's phone to use your VOIP connection.

Some VOIP systems allow people to walk up to a telephone instrument in any office and

dial a couple of numbers on the keypad ("log in"). That telephone then temporarily

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assumes the configuration of the telephone line in your home office. By assuming your 

telephone number, your list of speed dial numbers, and your intercom settings, it becomes

a temporary substitute for your VOIP phone in your office!

What is an Intranet ?

What is an intranet ?, What's the difference between intranet and internet ? 

A basic definition of Intranet an Intranet can be viewed as a small Internet which resides oncompany or private LANs or local area networks.

Intranets can be designed and built to suit the needs of all companies or home users, big or small, basic or complex, as in many large corporations which are used to encompass everythingfrom Human Resources information, to Payroll, to service desks to projects.

The Internet is quite literally open to everyone with an internet connection, an Intranet usuallyresides on a LAN or local area network, and as such is totally separate from the Internet.

Intranets operate in a secure area and their content can only be seen by people within the thisLAN or secure area. 

The purpose of an Intranet is to store any information which can be shared amongst.

Security on the internet is primarily on the web server itself, as opposed to security on Intranetswhich can be both web server security or integrated into domain security.

Corporate networks generally force you to use proxy servers to connect to the Internet, but not tothe Intranet.

Generally an Intranet will be faster than the Intranet, due to local servers and less users.

Similarities Between Intranet and Internet 

Both the Intranet and Internet operate on the same technologies, primarily HTML (Hyper TextMarkup Language) and TCP/IP (Transmission Control Protocol / Internet Protocol).

Both are there to collectively share information.

If you’re looking into an Intranet, contact consultants who specialize in developing andimplementing Intranets, who will be able to advise you towards taking the next step, where youcan learn about the benefits for your business.

The 7 Layers of the OSI Model

The OSI, or Open System Interconnection, model defines a networking

framework for implementing protocols in seven layers. Control is

passed from one layer to the next, starting at the application layer in

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one station, proceeding to the bottom layer, over the channel to the next

station and back up the hierarchy.

 

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next,

starting at the application layer in one station, and proceeding to the bottom layer, over 

the channel to the next station and back up the hierarchy.

Application (Layer 7)

This layer supports application and end-user processes. Communication partners are

identified, quality of service is identified, user authentication and privacy are considered,and any constraints on data syntax are identified. Everything at this layer is application-

specific. This layer provides application services for file transfers, e-mail, and other 

network  software services. Telnet and FTP are applications that exist entirely in theapplication level. Tiered application architectures are part of this layer.

Presentation (Layer 6)

This layer provides independence from differences in data representation (e.g.,

encryption) by translating from application to network format, and vice versa. The

 presentation layer works to transform data into the form that the application layer canaccept. This layer formats and encrypts data to be sent across a network , providing

freedom from compatibility problems. It is sometimes called the syntax layer.

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Session (Layer 5)

This layer establishes, manages and terminates connections between applications. The

session layer sets up, coordinates, and terminates conversations, exchanges, and

dialogues between the applications at each end. It deals with session and connectioncoordination.

Transport (Layer 4)

This layer provides transparent transfer of data between end systems, or hosts, and is

responsible for end-to-end error recovery and flow control. It ensures complete datatransfer.

Network (Layer 3)

This layer provides switching and routing technologies, creating logical paths, known asvirtual circuits, for transmitting data from node to node. Routing and forwarding are

functions of this layer, as well as addressing, internetworking, error handling, congestion 

control and packet sequencing.

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Data Link (Layer 2)

At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow

control and frame synchronization. The data link layer is divided into two sub layers: The

Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MACsub layer controls how a computer on the network gains access to the data and permissionto transmit it. The LLC layer controls frame synchronization, flow control and error 

checking.

Physical (Layer 1)

This layer conveys the bit stream - electrical impulse, light or radio signal -- through thenetwork at the electrical and mechanical level. It provides the hardware means of sending

and receiving data on a carrier, including defining cables, cards and physical aspects. Fast

Ethernet, RS232, and ATM are protocols with physical layer components.

Wi-Fi

The name of a popular wireless networking technology that uses radio waves to provide

wireless high-speed Internet and network connections. The Wi-Fi Alliance, theorganization that owns the Wi-Fi (registered trademark) term specifically defines Wi-Fi

as any "wireless local area network (WLAN) products that are based on the Institute of 

Electrical and Electronics Engineers' (IEEE) 802.11 standards.

Wi-Fi works with no physical wired connection between sender and receiver by usingradio frequency (RF) technology, a frequency within the electromagnetic spectrum

associated with radio wave propagation. When an RF current is supplied to an antenna,

an electromagnetic field is created that then is able to propagate through space. Thecornerstone of any wireless network is an access point (AP). The primary job of an access

 point is to broadcast a wireless signal that computers can detect and "tune" into. In order 

to connect to an access point and join a wireless network, computers and devices must beequipped with wireless network adapters (See "  How

WiMAX

WiMAX (Worldwide Interoperability for Microwave Access) is a telecommunications 

 protocol that provides fixed and mobile Internet access. The current WiMAX revision provides up to 40 Mbit/s[1][2] with the IEEE 802.16m update expected to offer up to 1

Gbit/s fixed speeds. The name "WiMAX" was created by the WiMAX Forum, which was

formed in June 2001 to promote conformity and interoperability of the standard. Theforum describes WiMAX[3] as "a standards-based technology enabling the delivery of last

mile wireless broadband access as an alternative to cable and DSL".[4]

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Terminology 

WiMAX refers to interoperable implementations of the IEEE 802.16 wireless-networks

standard (ratified by the WiMAX Forum), in similarity with Wi-Fi, which refers tointeroperable implementations of the IEEE 802.11 Wireless LAN standard (ratified by

the Wi-Fi Alliance). The WiMAX Forum certification allows vendors to sell their 

equipment as WiMAX (Fixed or Mobile) certified, thus ensuring a level of interoperability with other certified products, as long as they fit the same profile.

WiMAX is sometimes referred to as "Wi-Fi on steroids"[6] and can be used for a number 

of applications including broadband connections, cellular backhaul, hotspots, etc. It is

similar to Wi-Fi but it can also permit usage at much greater distances. WiMax is moreeffective on a larger scale and it is more cost-effective because the cost of moving

traditional broadband services to the next is more expensive. [1]

WiMAX adopted most of its technology from WiBro. However, WiBro is no longer used

for terminology since it

Uses

The bandwidth and range of WiMAX make it suitable for the following potential

applications:

• Providing portable mobile broadband connectivity across cities and countries

through a variety of devices.

• Providing a wireless alternative to cable and DSL for "last mile" broadband

access.

• Providing data, telecommunications (VoIP) and IPTV services (triple play).

• Providing a source of Internet connectivity as part of a business continuity plan.

Connecting to WiMAX 

A WiMAX USB modem for mobile internet

There are numerous devices on the market that provide connectivity to a WiMAX

network. These are known as the "subscriber unit" (SU).

There is an increasing focus on portable units. This includes handsets (similar to cellular 

smartphones); PC peripherals (PC Cards or USB dongles); and embedded devices inlaptops, which are now available for Wi-Fi services. In addition, there is much emphasis

 by operators on consumer electronics devices such as Gaming consoles, MP3 players and

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similar devices. It is notable that WiMAX is more similar to Wi-Fi than to 3G cellular 

technologies.

The WiMAX Forum website provides a list of certified devices. However, this is not acomplete list of devices available as certified modules are embedded into laptops, MIDs

(Mobile Internet devices), and other private labeled devices.

[edit] Gateways

WiMAX gateway devices are available as both indoor and outdoor versions from severalmanufacturers. Many of the WiMAX gateways that are offered by manufactures such as

Airspan, ZyXEL, Huawei, Motorola, and Greenpacket are stand-alone self-install indoor 

units. Such devices typically sit near the customer's window with the best WiMAX

signal, and provide:

• An integrated Wi-Fi access point to provide the WiMAX Internet connectivity to

multiple devices throughout the home or business.• Ethernet ports should you wish to connect directly to your computer or DVR  

instead.

• One or two PSTN telephone jacks to connect your land-line phone and take

advantage of VoIP.

Indoor gateways are convenient, but radio losses mean that the subscriber may need to besignificantly closer to the WiMAX base station than with professionally-installed external

units.

Outdoor units are roughly the size of a laptop PC, and their installation is comparable to

the installation of a residential satellite dish. A higher-gain directional outdoor unit willgenerally result in greatly increased range and throughput but with the obvious loss of 

 practical mobility of the unit.

Dongles

There are a variety of USB dongles on the market which provide connectivity to aWiMAX network. Generally these devices are connected to a notebook or netbook whilst

on the go. Dongles typically have omnidirectional antennae which are of lower-gain

compared to other devices, as such these devices are best used in areas of good coverage.

[edit] Mobile phones

HTC announced the first WiMAX enabled mobile phone, the Max 4G, on Nov 12th

2008.[13] The device was only available to certain markets in Russia on the Yota network.

HTC and Sprint Nextel released the second WiMAX enabled mobile phone, the EVO 4G,

March 23, 2010 at the CTIA conference in Las Vegas. The device, made available onJune 4, 2010,[14] is capable of both EV-DO(3G) and WiMAX(4G) as well as simultaneous

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data & voice sessions. The device also has a front-facing camera enabling the use of 

video conversations.[15] A number of WiMAX Mobiles are expected to hit the US market

in 2011.

Comparison with Wi-Fi

Comparisons and confusion between WiMAX and Wi-Fi are frequent because both are

related to wireless connectivity and Internet access.[25]

• WiMAX is a long range system, covering many kilometres, that uses licensed or unlicensed spectrum to deliver connection to a network, in most cases the

Internet.

• Wi-Fi uses unlicensed spectrum to provide access to a local network.

• Wi-Fi is more popular in end user devices.• Wi-Fi runs on the Media Access Control's CSMA/CA  protocol, which is

connectionless and contention based, whereas WiMAX runs a connection-

oriented MAC.• WiMAX and Wi-Fi have quite different quality of service (QoS) mechanisms:

o WiMAX uses a QoS mechanism based on connections between the base

station and the user device. Each connection is based on specificscheduling algorithms.

o Wi-Fi uses contention access - all subscriber stations that wish to pass data

through a wireless access point (AP) are competing for the AP's attentionon a random interrupt basis. This can cause subscriber stations distant

from the AP to be repeatedly interrupted by closer stations, greatly

reducing their throughput.

• Both 802.11 (which includes Wi-Fi) and 802.16 (which includes WiMAX) define

Peer-to-Peer (P2P) and ad hoc networks, where an end user communicates tousers or servers on another Local Area Network (LAN) using its access point or 

 base station. However, 802.11 supports also direct ad hoc or peer to peer networking between end user devices without an access point while 802.16 end

user devices must be in range of the base station.

Although Wi-Fi and WiMAX are designed for different situations, they are

complementary. WiMAX network operators typically provide a WiMAX Subscriber Unitwhich connects to the metropolitan WiMAX network and provides Wi-Fi within the

home or business for local devices (e.g., Laptops, Wi-Fi Handsets, smartphones) for 

connectivity. This enables the user to place the WiMAX Subscriber Unit in the best

reception area (such as a window), and still be able to use the WiMAX network from any place within their residence.

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Reference:

1. www.howstuffworks.com

2. www.discussionsworld.com3. www.google.com