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Appendix C: Overview of IP Routing IP Addresses & Subnets - Page 125
Appendix C: Overview of IP Routing
IP Addresses & Subnets
Each computer/host is given a unique number or “IP Address”. The address is 32
bits long e.g. 11000000101010000010101000000001.
This is represented by splitting it into 4 groups of 8 bits and convert them from
binary into decimal numbers
Thus 11000000101010000010101000000001
becomes 11000000 10101000 00101010 00000001
becomes 192 168 42 1
We then add dots between the numbers to make them easily recognisable as an
IP address, e.g.. 192.168.42.1.
The computers or hosts communicate by putting data into packets and labelling
them with the source and destination IP addresses. When computers
communicate they do not care where the destination is their task is simply to
pass the packet to the next machine and then forget about it. To keep life simple
all the computers/hosts in an office are given similar numbers e.g. 192.168.42.1,
192.168.42.2, 192.168.42.3, etc. The Router is the “gateway” to the rest of the
world and its job is to cope with that traffic. This makes it easy for each computer
to decide whether to send the packet either directly to another local machine or
the “gateway” using the subnet mask.
A computer uses the subnet mask, i.e. 255.255.255.0, to decide if a packet is for
the router or the LAN.
The computer does a binary AND with its own address and the subnet mask then
the destination AND the subnet mask if the result is not the same then the packet
is for the router.
For example: You have a Router 192.168.42.1, and the following PCs are
communicating with each other:
– PC 'A': 192.168.42.201.
– PC 'B': 192.168.42.202.
– PC 'C': 158.152.1.43.
– PC 'A' to PC 'B':
Source IP address 192.168.42.201, subnet mask 255.255.255.0 ANDing
gives 192.168.42.0. Destination 192.168.42.202, subnet mask 255.255.255.0
ANDing gives 192.168.42.0 which is the same so both computers are on
same LAN
– PC 'A' to PC 'C': Source IP address 192.168.42.201, subnet mask
255.255.255.0 ANDing gives 192.168.42.0. Destination 158.152.1.43, subnet
mask 255.255.255.0 ANDing gives 158.152.1.0 which is different so the
packet is sent to the router.
So for basic operation of a computer you need
– An IP address: e.g.. 192.168.42.201
– An subnet mask: e.g.. 255.255.255.0
– A Gateway address: the IP address of the router, e.g.: 192.168.42.1.
There are special IP addresses called broadcast addresses which are seen by all
computers on a LAN e.g. 255.255.255.255 or 192.168.42.255
INDeX IPNC Cassette Administration Manual Appendix C: Overview of IP Routing - Page 125
38DHB0002UKDD – Issue 7 (22/11/02) IP Addresses & Subnets