A network mask is used when a network is not sub netted. The subnet mask is used to find the first address in the block. However when a mask is sub netted, the situation is different. We must have a subnet mask. The subnet has more 1s.

Subnetting increases the length of the netid and decrease the length of hostid. When we divide a network to s number of subnetworks, each of number of hosts, we can calculate the subnetid for each subnetwork as in which n is the length of netid,.

When a network is subnetted, the first address in the subnet is the identifier of the subnet and is used by the router to route the packets destined for that subnet works. Given any address in the subnet, the router can find the subnet mask using the same procedure we discussed to find the subnetwork mask. ANDing the given address with the subnet mask.

Subnetting couldn’t completely solve address depletion problems in classful addreeing, because most organization did not want to share their granted blocks with others. since class C blocks were still available but the size of block did not meet the requirement of new organization that wanted to join the internet, one solution was super netting . in super netting, an organization can combine several class C block to create a larger range of addresses. In other, words several networks are combined to create a supernet work. By doing this, an organizarion can apply for several class C blocks instead of just one. For example, an organization that needs 1000 addresses can be granted four class C blocks.