A specification of which part of an IP address (see TCP/IP) represents the network identity. It is a 32-bit bitmap where ‘1’ means that bit position is part of the network identity and ‘0’ that it is part of the host address. This bitmap always take the form of a single block of ‘1’s (the network identity part) followed by a single block of ‘0’s (the host address part). It is important that routers can distinguish efficiently between those packets originating on its network whose destination is on the same network and those that that require routing over an inter-network. This operation requires isolating the network identity part of the IPA address, and an AND operation with the subnet mask is a very efficient way to do this. For convenience, subnet masks are usually represented either as the decimal form of four 8-bit numbers or as an integer representing the network identity. Thus, 255.255.255.0 and /24 both indicate a 24-bit network identity and an 8-bit host address.