An address mask is a term used in computer networking that describes the portion of an Internet Protocol (IP) address that will represent the subnet. The most common name for this is the subnet mask or bit mask. The address mask is represented in binary format, meaning in 1s and 0s.
To truly understand an address mask, one must understand what a subnet is and how an IP address or network address works. Typically, an IP address will have either an 8-, 16-, or 24-bit network ID that is used to distinguish between different computers on a network. A subnet allows for those bits to be extended, so instead of having only one IP address or network address, there can be two or three; this is called subnetting. In basic terms, a subnet is a network that falls within one of the three types of IP addresses: Class A, which is the 8-bit network; Class B, which is the 16-bit network; or Class C, which is the 24-bit network.
For subnetting to work, a router has to know which part of the host ID — the computer that is on the network — will be used by the network ID. This is where the address mask comes in. Computers talk to each other in binary, that is with 1s and 0s. On a network, the router looks for both the IP address and the address mask. It then performs a complex operation in which it discovers the network ID. A network ID may look something like 192.168.2.0.
Subnet masks are determined by the IP address class, that is the Class A, Class B, or Class C variety. For most private networks, including the well-known Internet service providers, the subnet mask will be something along the lines of 255.255.255.0, which puts that particular mask within the class C networks. An address mask isn't necessarily needed because the IP address class helps determine what the mask should be. If the first bit is 0, the mask is a part of Class A; if the first two bits are 10, then it is part of Class B; and if the first three bits are 110, then it is Class C.
Address masks are always composed of bits that are set to 1, so only nine values are possible for the address: 0, 128, 192, 224, 248, 252, 254, and 255. The minimum number for a network ID is eight bits, so the first eight bits of a subnet mask will always be 255.