|  | 10.7 | Creating a Subnet
| 10.7.4 | Computing hosts per subnetwork

Each time you borrow 1 bit from a host field, there is 1 less bit remaining in the field that can be used for host numbers. Specifically, each time you borrow another bit from the host field, the number of host addresses that you can assign decreases by a power of 2 (gets cut in half).

To help you understand how this works, use a Class C network address as an example. If there is no subnet mask, all 8 bits in the last octet are used for the host field. Therefore, there are 256 (28) possible addresses available to assign to hosts (254 usable addresses, after you subtract the 2 you know you can't use). Now, imagine that this Class C network is divided into subnets. If you borrow 2 bits from the default 8 bit host field, the host field decreases in size to 6 bits. If you write out all of the possible combinations of 0s and 1s that could occur in the remaining 6 bits, you would discover that the total number of possible hosts that could be assigned in each subnet would be reduced to 64 (26). The number of usable host numbers would be reduced to 62.

In the same Class C network, if you borrow 3 bits, the size of the host field decreases to 5 bits and the total number of hosts that you could assign to each subnet would be reduced to 32 (25). The number of usable host numbers would be reduced to 30.

The number of possible host addresses that can be assigned to a subnet is related to the number of subnets that have been created. In a Class C network, for example, if a subnet mask of 255.255.255.224 has been applied, then 3 bits (224 = 11100000) would have been borrowed from the host field. The useable subnets created are 6 (8 minus 2), each having 30 (32 minus 2) useable host addresses.

Exercise:
Divide the last octet into two parts: a subnet field and a host field. If there are 32 possible host addresses that can be assigned to each subnet, then their IP addresses would fall within the range of numbers (but remember the 2 unusable host addresses in each subnet!).

In a Class C network 199.5.12.0 with subnet mask 255.255.255.224, to which subnet would host 199.5.12.97 belong? (hint: 97 = 01100001 binary)

1. subnet 0?
2. subnet 1?
3. subnet 2?
4. subnet 3?
5. subnet 4?
6. none of the above?