The number of hosts supported by a network identifier is calculated from the number of zeros in the assigned subnet mask. If the number of zeros is denoted by n, the number of hosts is calculated as 2n-2. The two addresses that are subtracted in the expression represent the reserved IP addresses that cannot be assigned to any host.
The bits in a host identifier cannot all have the value 0, nor can they all have the value 1. The host identifier with the value 0 is used to designate a network identifier. The host identifier consisting only of ones is used as an IP broadcast address. The following tables show an example of a valid host identifier and an example of a reserved host identifier.