Lecture 9: The IP Protocol


The IP Protocol

Recall earlier: the TCP (transport layer) protocol is implemented in the two end systems involved in communications.

TCP relies on the Internet Protocol to deliver packets of data from one end system to another. The packets which IP delivers are usually called datagrams.

Datagrams navigate zero or more (sometimes many) routers, interconnecting the individual networks which together make up the Internet. The word "Internet" is just a contraction of the phrase "Interconnected Networks".

A router is essentially a special-purpose computer whose sole function is to transfer datagrams between networks. In earlier times, they were called "IP Gateways". The general structure of the Internet can thus be visualised:

Router-connected networks

IP Addressing

The concept of a global Internet addressing scheme is fundamental to IP.

Every Internet-connected system has a unique Internet host address. This is a 32 bit, or 4 byte, binary number.

Internet addresses are written as a dotted sequence of the form:

aa.bb.cc.dd
where aa, bb, etc, are the decimal values (ranging from 0 to 255) of the 4 bytes which make up the internet address, for example:
149.144.21.60
This is called the "Common Internet Address Notation". Internet addresses are commonly referred to as "IP addresses".

The dotted sequence 149.144.21.60 is the IP address of "ironbark", or to use its full name, ironbark.bendigo.latrobe.edu.au.


"Two Part" Addresses

The 4 byte internet address consists of 2 parts, the network part and the host part.


Address Classes

The structure of an IP address differs depending on the class of the local network to which the host is connected:

Class A Network
the first (or leftmost) field, or byte, of the address specifies the network number and class, and can take values from 1 to 126.

Class B network
the leftmost two fields (bytes) specify the network number and class. The first field is in the range from 128 to 191, and the second from 1 to 254.

Class C network
the leftmost three fields specify network number and class. The first field can be from 192 to 223, the second from 0 to 255, and the third from 1 to 254.


More On Address Classes

The Internet address format can be described graphically:
IP address classes
Example 1:
in a class A network, only values from 00000000 to 01111111 are possible. Of these 128 combinations, 126 are permitted (networks 0 and 127 are reserved for other purposes). Hence there can only ever be 126 class A networks, each of which can have a huge number of hosts.

Example 2:
ironbark's IP address is 149.144.21.60 This is a class B network (149.144), and ironbark's host ID is 21.60

Example 3:
203.17.3.5 is a class C address. The network number is 203.17.3, and the host number is 5.

Network numbers above 223 are also reserved for special purposes, outside the scope of this subject.


IP Broadcast Address

The broadcast address is the address used to send a datagram to all hosts on the local network simultaneously. This is the address given by the network number followed by all binary 1's in the host part of the address.

For example: The broadcast address for a host on the La Trobe, Bendigo Class-B IP network would be[1]:

149.144.255.255
An example class C broadcast address is:
197.23.114.255
[1] Except for the fact that subnetting is in use, see next slide.

IP Subnetting

Where Class-B networks (and, nowadays, class C also) are in use, the host part of the address is commonly split, for management purposes, to allow subnetting -- typically this involves defining 254 subnets each of 254 hosts. Each of these subnets behaves, for all intents, like an independent class-C network: thus:
IP subnet address format
Exactly which bits of the subnetted address are used for the network/subnet part and the host part is defined using an address mask, or netmask thus:
255.255.255.0
For example, at Bendigo the "ironbark" UNIX system address is:
Address = 149.144.21.60 Netmask = 255.255.255.0
Thus ironbark is addressed as:
class B network: 149.144
subnet: 21
host number: 60

This lecture is also available in PostScript format. The tutorial for this lecture is Tutorial #08.
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