sub-netting means we r going to divide a single network to
many different sub network by borrow host bit as a network
bits in subnet mask. after subnetting the default subnet
mask has been increasing.(but have same prefixes)  

basically when we have a single network address and we need
more than one network address for security, network
broadcast Strom, load balance, remote sites etc. reason then
we are going for subnetting.