addressing the ‘what’ and ‘where’ of communication

Addressing

The ‘What’ and ‘Where’ of Communication

Addressing• Addressing is necessary for any communication

– To talk: Appearance, name, …– To call: Telephone numbers– To mail: Postal address– To visit: Postal address + directions– To E-Mail: E-Mail addresses– To instant message: ICQ#, AIM ID, etc.

• These ‘addresses’ allow us to uniquely identify the entity with which we wish to communicate

Addressing a la Shoch• Name/Identifier: What

– Names normally identify the entity– If an entity moves, the name/identity will remain the same

• Address: Where– Addresses identify the location of the entity– If an entity moves, the address will change

• Route: How to get there– Routes identify the path to get to an entity– If an entity moves, the route will change

Addressing

• Addressing deals with how to define an entity’s location (uniquely)

• Addressing is necessary for message delivery– An address is the start and end point for the route

• However, routing is another subject

– Where do we want the message to go?

Addresses• We have already seen MAC addresses (for Ethernet

and some other LANs):– e.g. 02-60-8C-08-E1-0C– 6 octet address– Globally unique– Defined statically by the hardware manufacturer

• Most people are familiar with the IP addresses used by TCP/IP networks:– e.g. 137.207.32.2– 4 octet address– Not necessarily globally unique– Defined dynamically by DHCP servers or negotiated by the

operating system

IP Addressing

A Closer Look

IP Addresses

• TCP/IP networks use IP for the network layer protocol

• IP defines 4 octet addresses– 4 billion possible addresses

• Usually written in the form A.B.C.D– A, B, C, and D are each 1 octet (0-255), normally written in

decimal notation– Thus, IP addresses fall in the range:

0.0.0.0 – 255.255.255.255

IP Addresses

• Originally intended for separate internets (interconnected LANs)– Thus, the 32 bit size was not a concern– 48 bits is generally considered a fairly safe size for

globally unique addressing– Computers connected to ARPANET (and later

incarnations) were just given consecutive addresses

1.0.0.0, 1.0.0.1, 1.0.0.2, …

IP Addresses

• Any computer connected to a TCP/IP network (e.g. the Internet) must have an IP address

• Further, any network interface card (NIC) using TCP/IP to access an network (e.g. the Internet) must have a different IP address

IP Addresses

• Even though there are 4 billion possible IP addresses, they are running out

• Here’s why:– Some of the bits are dedicated to header

information (discussed later)• ½ the addresses for each lost bit

– Addresses are categorized, and some of the categories are running out of addresses (while others are not)

Non-Classed Addresses

• Part of the address represented the network the computer resided on, and part represented the computer itself– Network: 7 bits (up to 128 networks)– Computer: 24 bits (up to 1.6 million computers on each

network)

• Since there were very few networks on ARPANET originally, this wasn’t a problem

Address Classes

• When private organizations started joining the Internet, the needs became obvious– Some (fewer) networks have multitudes of

computers (thousands)• e.g. The @Home network

– Some (many) networks have very few computers (a few hundred or less)

• e.g. The Windsor Police Department

Address Classes

• Quickly, the addresses were separated into 3 classes (plus room for more classes if needed):– Class A: Fewer networks, many nodes– Class B: Medium networks, medium nodes– Class C: Many networks, fewer nodes

IP Address Classes

00

bit index: 1-7network

8-31host (machine)

Class A:

Class B:0

02-15

network16-31host1

1bit index:

Class C:0

13-23

network24-31host1

1bit index:02

IP Address Classes• Class A:

– Range: 1.0.0.0 – 126.0.0.0– Networks: 128 max, Machines: 65537-1.6 million– e.g. huge networks, such as large military/government

organizations (e.g. FBI), the @Home network, etc…• Class B:

– Range: 128.1.0.0 – 191.255.0.0– Networks: 16384 max, Machines: 257-65536– e.g. Internet service providers (ISPs) (dial-up)

• Class C:– Range: 192.1.0.0 – 223.255.255.0– Networks: 2 million max, Machines: 1-256– e.g. Small businesses

IP Address Classes

• The IP address classes are self-identifying– Which means that given the address, you can determine

what class an address is• Actually, using only the first number

– Examples:• 137.207.32.2 (server.uwindsor.ca)

– 137 -> Class B• 24.0.0.1 (@Home DHCP server)

– 24 -> Class A

Other IP Address Classes

0 1 2 31 1 1 0

bit index:Multicast group address

4-31Class D:

Class E:0 1 2 3 4 5-31

Reserved for future use1 1 1 1 0bit index:

•These addresses are used to represent multicast groups•Discussed later

•These addresses were left open to be used and divided into classes as needed

Special IP Addresses• 0.0.0.0: Used to indicate that this machine is without an

assigned IP– Used during bootstrapping (e.g. requesting an IP from a DHCP server)

• <all 0s (binary)><hostID>: Used to send messages to some machine on this network

• 255.255.255.255: Used to send broadcast messages across this machine’s network

• <netID><all 1s (binary)>: Used to send broadcast messages to the specified network

• 127.0.0.1: Used to send messages back to this machine (called loopback or localhost)

IP Addressing Comments

• In IP addressing:– 0’s usually represent ‘this’– 1’s usually represent ‘all’

• Broadcasting, although discussed here in terms of addressing, will be discussed further

Loopback

• The 127.0.0.1 address, does not normally exist on the network– Either as the source address or destination address of a

packet• The address is used internally by NICs

– When a NIC receives a message addressed with 127.0.0.1 to be transmitted, it passes the message directly to the receiver hardware

– The receiver hardware returns the message to the operating system exactly as if the message were received from the network

• However, the message never entered the network medium

Internal IP Addresses

• Depending on the address class needed by an organization, a range of internal addresses is available:– Class A: 10.0.0.0 – 10.255.255.255– Class B: 172.16.0.0 – 172.31.255.255– Class C: 192.168.0.0 – 192.168.255.255

• IP routers outside a private (connection-shared) network, will not forward datagrams designated for addresses in these ranges

Multi-homed Machines

• There is no restriction preventing machines from participating in multiple networks– A machine could have multiple NICs– Each NIC would have its own MAC address– On TCP/IP networks, each of these NICs would be

given a different IP address


Class B private network

Class C private network

172.16.0.1 172.16.0.2 172.16.0.3 172.16.0.4

172.16.3.14172.16.3.15172.16.3.16172.16.3.17

172.16.3.18

192.168.0.1 192.168.0.2 192.168.0.3

192.168.0.4

192.168.0.5192.168.0.6192.168.0.7

192.168.0.8

MMM

M

M

M

M

M

MMMMMMMM


Class B private network

Class C private network

172.16.0.1 172.16.0.2 172.16.0.3 172.16.0.4

172.16.3.14172.16.3.15172.16.3.16172.16.3.17

172.16.3.18

192.168.0.1 192.168.0.2 192.168.0.3

192.168.0.4

192.168.0.5192.168.0.6192.168.0.7

192.168.0.8

Routers• Routers are multi-homed machines

– They have a number of network ports, each of which represents a different path

• Routers use tables that relate destinations to network paths– Internet routers relate destination network addresses with

one of their network ports– When a datagram arrives at a router:

• Its destination address is used to determine the network address• The network address is used to look up the destination port in the

routing table

Network Addresses• An IP address can be used to calculate the address of

the network• The machine address is passed through a filter

(called a subnet filter):– This filter extracts the bits of the address that represent

the network and sets the bits that represent the machine to zero

– The filter determines which part of the address represent the network address, by using the subnet mask

Subnet Mask• The subnet mask is a binary number, that has 0s in

the machine portion of the address, and 1s in the network portion

• Most networks of each type use a constant subnet mask– Class A: 255.0.0.0

(Binary: 11111111000000000000000000000000)– Class B: 255.255.0.0

(Binary: 11111111111111110000000000000000)– Class C: 255.255.255.0

(Binary: 11111111111111111111111100000000)

Using Subnet Masks

• Example:– Address: 137.207.32.2– Subnet Mask: 255.255.0.0

Address: 10001001110011110010000000000010Mask: 11111111111111110000000000000000Net Address: 10001001110011110000000000000000

• Network address: 137.207.0.0

Routing in Action

Network N1 (Class B)Address: 137.207.0.0

Network N2 (Class C)Address: 194.201.61.0

Network N3 (Class A)Address: 24.0.0.0

Internet

137.207.0.0 P1

194.201.61.0 P2

24.0.0.0 P3

* P4

P4P1

P2 P3

IPv6

Next Generation Addressing in TCP/IP Networks

IPv6

• Due to the limited nature of existing IP addressing (IPv4), a new version of IP addressing was developed

• This new scheme uses 16 octets for addresses, instead of 4 octets

• Written using hex notation:

3A57:0000:0000:9CD5:3412:912D:6738:1928

IPv6 Features

• 16 octet addresses (128 bits)• Larger numbers of address classes

– More accurate control of network/machine counts

• Variable-sized headers– Optional information can be placed into the header

when needed– Reduces header size in most cases

• Extendible protocol– IPv6 allows for new header information to be added to

support different protocols

IPv6 Features

• Automatically reconfigurable– Addresses can be automatically reassigned dynamically– e.g. when a certain number of nodes join the network,

a different address class may be desired

• Autoconfigurable– The use of autoconfiguration (such as DHCP) allows

dynamic private addressing and dynamic public addressing

IPv6 Datagram Format

header extension headers data

optional

IPv6 Header Format

version traffic class flow label

0 4 12 31

payload length hop limitnext header

32 48 56 63

source address destination address

64 96 128

IPv6 Integration• Will IPv6 replace IP addresses?

– Who knows?• Currently, temporary solutions have made IPv4

addresses capable of lasting longer than originally predicted

• If and when IPv6 is to be integrated, the process must be a transition– Closing the entire Internet down to convert hardware

and software to IPv6 not going to happen– Some stations may take longer to transition than other

stations• e.g. Bob’s Internet Shack vs. the Telus Network

IPv6 Integration• NAT (network address translators) provide one example

of such a temporary solution• NATs provide three benefits:

1. NATs provide IP masquerading• Messages using these addresses pass through a network address

translator (NAT) to be transformed into external IPs

2. NATs provide IP sharing• ISPs for example, have many customers, but significantly less at

any given time are logged onto their system– IP addresses can be assigned dynamically to these customers when

they log in

3. NATs provide schemes to allow networks to use either IPv4 or IPv6

– Addresses would be converted as they pass through a NAT

IPv6 Integration

• Another method that may be used for the transition between IPv4 and IPv6 is address inclusion:

– IPv4 addresses could be embedded into IPv6 addresses

• Translation between the two types of addresses is possible without any other information

– Some problems exist with this approach, but in general it simplifies communication between IPv6 networks and IPv4

Special IPv6 Addresses• 0:0:0:0:0:0:0:0 Used to indicate that this machine is

without an assigned IP– Used during bootstrapping (e.g. requesting an IP from a

DHCP server)• 0:0:0:0:0:0:0:1 Used to send messages back to this

machine (called loopback)– These two addresses are not valid on the actual network

medium (same as with IPv4)• 00:… Reserved (including IPv4 and IPX address

inclusion)• FF:… Multicast addresses

IPX

Internetwork Packet Exchange Addresses

IPX

• IPX was originally created to replace IP• In reality, it is used primarily on LANs• In conjunction with the SPX protocol, formed

one of the two protocol suites used in Netware networks– SPX is to TCP, what IPX is to IP

• Still can be (although rarely is) used today in Windows networks

IPX Addresses

• IPX uses a 2 component address (like IP):– The network portion (4 octets)– The machine portion (6 octets)

• Unlike IP, these sizes are constant– So there are no IPX address classes

• IPX uses sizes large enough to accommodate all categories of networks

IPX Addresses

• The network portion of an IPX address is 4 octets (32 bits)– This allows for 4.29*109 networks (4 billion)– This is almost enough for everyone on earth to have their

own network

• The machine portion of an IPX address is 6 octets (48 bits)– This allows for 2.81*1014 machines on each network

(281,475 trillion)

IPX Addresses

• The scalability of IPX addresses is not their only benefit

• The constant size of the network and machine address portions simplifies extracting each portion– As a result, machines that process IPX networks

can process IPX datagrams more quickly• Such as network nodes, routers, etc.

IPX Addresses

• Why 48 bits for the machine portion?– 48 bits allows for way too many machines, more than will

be needed for many years• By the time machine IDs run out, network hardware and software

will have been obsolete by many years!

– Using 48 bits allows hardware to use the machine’s MAC address as the machine portion

• This makes auto-configuration (dynamic IPX address assignment) easier/faster

IPX Addresses• If IPX has been around since the NetWare days, why

don’t we use it for the Internet, instead of IP?– Good question! Why don’t we?– Frankly, IPX has a bad reputation, because initially it was

used with SPX and other bandwidth-munching protocols– IPX can be used in conjunction with TCP (TCP/IPX), and it

would make an excellent replacement for TCP/IP• However, standards organizations (e.g. ISO) want to use the

protocols they develop, and not ones developed by corporations, such as Novell

Fragmentation & Reassembly

• Packets can arrive out of order in connectionless networks

• Packets must be reordered during reassembly• During fragmentation, the portion of data that each

fragment represents must be identified– Since the length of a packet’s data can always be

determined, all that is necessary is to use the offset of the start of the packet’s data in the larger data chunk


Logical Data Chunk

Packet 1 Packet 2 Packet 3 Packet 4 P5

0 1500 3000 4500 6000 6800

Offset:0 Offset:1500 Offset:3000 Offset:4500 Offset:6000Length:1500 Length:1500 Length:1500 Length:1500 Length:800


• Why do we use the data offset, and not just a sequence of numbers to determine packet order?– Sometimes, packets can be fragmented at one location,

and must be re-fragmented at another location (such as while passing through a network incompatible with larger frame sizes)

– These situations would require renumbering of all packets in the sequence, which is not always possible


Packet 1 Packet 2 Packet 3 Packet 4 P5

Offset:0 Offset:1500 Offset:3000 Offset:4500 Offset:6000Length:1500 Length:1500 Length:1500 Length:1500 Length:800

Packet 1 Packet 2 P3a Packet 4 P5

Offset:3000Length:1000

P3b

Offset:4000Length:500


• Re-fragmentation (at gateways, routers, …) is expensive– The re-fragmenting node must process each packet,

fragmenting it into smaller packets– Another reassembling node must collect these packets

and assemble them into larger packets– Each of these operations involved memory processing,

which is expensive when applied to many packets per second

Fragmentation & Reassembly• Re-fragmentation (particularly in routers) should be

avoided at all costs– To virtually eliminate re-fragmentation in a network, the

maximum transmission unit (MTU) should be determined and used as the packet size

– A network’s MTU is the largest size that can be used for packets that will not result in any re-fragmentation by any routers, or other multi-homed nodes

– Schemes for determining the MTU dynamically have been developed, but are beyond the scope of this course

IP Datagrams

• IP datagrams are packets sent over IP networks using connectionless messaging

• Datagrams can be used directly within network-capable programs by sending datagrams via UDP (user datagram protocol)

• Datagrams are used transparently by TCP to provide connection-based transport

IP Datagramsbits Name Description Used For4 Version Version (equal to 4) All

4 IHL Header length All

8 TOS Type of Service (obsolete) -

16 Length Total length of datagram (header included) All

16 ID Identifier: used in reassembly to identify packets Reassembly

1 DF Should the datagram be re-fragmented, if necessary? Routers (re-frag)

1 MF Are there more fragments in the sequence? Reassembly

13 Offset Offset of data that this datagram represents Reassembly

8 TTL Hop limit Routers

8 Protocol Transport protocol used for this packet (UDP, TCP) Acknowledgement

16 Checksum Checksum of the header All

32 SA Source address All

32 DA Destination address All

? Options Future features -

? Padding Fills remaining space -

IP Datagram Routing

• When an IP-enabled router receives a datagram, it:– Receives a datagram through one of its ports– Deletes the datagram, if the hop count (TTL in IPv4, Hop limit in

IPv6) has a non-positive value– If the hop count is positive, it is decremented and processing

continues– Determine the destination address’ network address– Uses the destination network address to find an entry in the routing

table– Uses the routing table entry to determine to which port the

datagram should be sent– Sends the datagram through the correct port

IPv6 Datagrams

bits Name Description Used For4 Version Version (equal to 6) All

8 TOS Type of Service (status info) All

20 Flow label Future features -

16 Length Length of data in the datagram (header not included) All

8 Hop limit Hop limit (decremented to zero) Routers

16 SA Source address All

16 DA Destination address All

? H2H Hop to hop header Routing

? SRH Source routing header Routing

? FH Fragment header Reassembly

? E2E End to end options Reassembly

Header Checksums

• Networks sometimes result in corrupt data• Information in the header is equally susceptible to this

corruption• However, header information, when corrupt, can cause more

serious difficulties– For example, the destination address may have a few bits changed, or

the hop count, etc.– Corruption like this, is not always easy to detect and fix– Corrupt data (determined by another checksum) can be fixed by re-

issuing the datagram– Header checksums are used to ease identification of header corruption

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addressing the ‘what’ and ‘where’ of communication

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