Cisco CCNA

Calculating the Number of Host Addresses in a Subnet

Calculating the number of hosts that can be addressed per subnet is not difficult. Each bit position can be either a one or a zero, so starting with one bit, there are two possible combinations. The number of possible combinations doubles each time you add an additional bit. Two bits yields four combinations, three bits yields eight combinations, four bits yields 16 combinations, and so on.

The formula for the number of combinations is 2", where n is the number of bits in the field. Example I -1 has five bits in the host field after three bits are stolen for the subnet field. This yields 25=32 unique combinations for addressing hosts; however, the all-zeros and all-ones patterns are reserved for the subnet number and subnet broadcast address, respectively. After subtracting these two reserved addresses, 30 addresses per subnet remain for host addresses.