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:
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.
- We shall see later how the name of a computer (ironbark,
redgum, bindi, etc) is mapped to its IP address.
- Note that the traditional Internet literature refers to
internet-connected computers as hosts, which reflects
the historical nature of computers as multi-user timeshared systems.
"Two Part" Addresses
The 4 byte internet address consists of 2 parts, the network part
and the host part.
- The network part is the same for all hosts connected to the same
network. The network part is used to route datagrams across the
Internet.
- Every network in the global Internet has a unique network
identification number.
- The host part must be unique within each specific network. It
is used to deliver the datagram once it has been routed to the
appropriate network.
- The combination of the unique network number plus a
"host-within-network" number means that every Internet-connected
computer has a globally unique IP address.
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:
- 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:
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.
[Previous Lecture]
[Lecture Index]
[Next Lecture]
Copyright © 2000 by
Philip Scott,
La Trobe University.
