An address mask, also known as a subnet mask or bitmask, represents the subnetwork of an IP address in binary format. Subnetting allows for multiple IP addresses under one network ID, determined by the class of the IP address. The router uses the address mask to determine the network ID. The first eight bits of a subnet mask are always 255.
An address mask is a term used in computer networking that describes the portion of an Internet Protocol (IP) address that will represent the subnetwork. The most common name for this is subnet mask or bitmask. The address mask is represented in binary format, i.e. in 1 and 0.
To truly understand an address mask, you need to understand what a subnet is and how an IP address or network address works. Typically, an IP address will have an 8-, 16-, or 24-bit network ID used to distinguish between different computers on a network. A subnet allows those bits to stretch, so instead of having just one IP address or network address, there can be two or three; this is called subnetting. Simply put, a subnet is a network that falls under one of three IP address types: Class A, which is the 8-bit network; Class B, which is 16-bit networking; or Class C, which is 24-bit networking.
For subnetting to work, a router needs to know what part of the host ID, i.e. the computer on the network, the network ID will use. This is where the address mask comes into play. Computers talk to each other in binary, i.e. with 1s and 0s. In a network, the router looks up both the IP address and the address mask. Then it does a complex operation where it discovers the network ID. A network ID can look something like 192.168.2.0.
Subnet masks are determined by the class of the IP address, which is the Class A, Class B, or Class C variety. For most private networks, including well-known Internet Service Providers, the subnet mask will be something along the lines of 255.255 .255.0, which puts that particular mask into 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 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’s Class C.
Address masks always consist of bits 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.
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