The key concept in subnetting is borrowing bits from the host portion of the network to create a subnetwork.Rules govern this borrowing, ensuring that some bits are left for a host ID. The rules require that two bits remain available to use for the host ID and that all of the subnet bits cannot be all 1s or 0s at the same time. For each IP address class, only a certain number of bits can be borrowed from the host portion for use in the subnet mask.
Remember - Subnetting, a subnet, and subnet mask are all different. In fact, the first creates the second and is identified by the third.
subnetting is the process of dividing a network and its IP addresses into segments, each of which is called a subnetwork, or subnet. The subnet mask is the 32-bit number that the router uses to cover up the network address to show shich bits are being used to identify the subnet.
Bits Available for Creating Subnets
| Address Class | Host Bits | Bits Available for Subnet |
| A | 24 | 22 |
| B | 16 | 14 |
| C | 8 | 6 |
Calculating Subnets
There are two simple formulas to calculate these numbers:
Number of hosts per subnet = ( 2 number of bits used for host ) - 2
Number of subnets = ( 2 number of bits used for subnets ) - 2