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Definition: a bit pattern, usually written as four numbers, that indicates which parts of an IP address belong to the same network. The most common subnet mask is 255.255.255.0, which indicates that the first three numbers of the IP address are the same throughout the network. An alternative on larger networks is 255.255.0.0, which indicates that only the first two numbers are the same. Here 255 means “the number in this position is the same throughout the network” and 0 means “this number varies from computer to computer.” More technically, subnet masks are 32-bit binary numbers which are logically ANDed with an IP address to extract the part that identifies the network.
Source: Downing, Douglas, et al. Dictionary of Computer and Internet Terms. 10th ed. New York, 2009. Print.