Splitting a Network Into Subnets

A single large network often needs dividing. Subnetting splits one address block into several smaller subnets to separate departments, improve security, and shrink broadcast domains.

You do this by borrowing host bits and turning them into network bits, increasing the prefix length.

To create subnets, you borrow bits from the host portion. Each borrowed bit doubles the number of subnets: borrow 1 bit for 2 subnets, 2 bits for 4, 3 bits for 8.

The formula is 2^(borrowed bits) = number of subnets. Borrowing bits also reduces the hosts available in each subnet.