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 |
