Explain about Subnet – The Meaning and Definition
The design of classful IP addressing has lead to several problems like each host in a network must have same network number, and single class A,B, or C Address refers to one network on the whole not to a collection of LANs. It is common to have several LANs, in a big institute an each LAN to have individual router or all such router to connect to internet through main router at institute level.
These individual parts of the network are called ‘subnets’. Instead of main router having one entry per machine. On the whole network, we can have each subnet’s router address in main router and each subnet’s router maintains individual host’s address. Thus, burden on routers comes down drastically.
To make above scheme work, some bits from host filled of IP addresses are interpreted as network, number. The number of such subnet bits should be in such a way that total number of combinations possible with those bit should be greater than number of subnets.
Now the main router needs ‘subnet mask’ to indicate network, subnet number, and host number on subnet pass of an IP address. Subnet masks are represented like IP addresses, in dotted decimal notation.
In the new scheme, each router only contains three fields, (this network, subnet, 0) or (this network, this subnet, host). Thus a router on a subnet knows how to reach other subnet and all host on its subnet. It need not know about all the hosts on other subnet. When a packet arrives at a router it is ANDed with the subnet mask. This address is looked up in the routing task for further forwarding.