Basic Concept

Basic Concept of Subnet

When we are subnetting, we are borrowing host bits. We never borrow network bits.
And the bits that we are borrowing are becoming our subnet bits.
For example, in a Class A network, the first 8 bits represents the network. Let we extend the network portion, first 11 bits represents the network and the rest represents the host.
So, we are taking out 3 bits from host area and we are appending it to the network part.
So, the 8 bits of the default mask of Class A network gets 3 additional bits from the host portion, we get a 11 bits network part.
Out of the network part 8 bits will be the network and 3 bits will be the sub
Network. This is the basic idea of subnetting.

There are two sample picture to understand SUBNET: