A 32-bit value that indicates the number of available host addresses on a subnet, which uses TCP/IP knowing that the first and last addresses are reserved (for example, subnet mask 255.255.192.0 allows 317 addresses for classless devices). In classful addressing, the subnet mask follows a convention as to the number of available hosts (for example, for a class C network, the subnet mask is 255.255.255.0, which allows 254 host addresses).