The subnet mask is a 32-bit number in the same format and representation as IP addresses. The subnet mask determines which bits in the IP address are interpreted as the network number, which as the subnetwork number, and which as the host number. Each IP address bit that corresponds to a 1 in the subnet mask is in the network/subnetwork part of the address. This group of numbers is also called the Network ID. Each IP address bit that corresponds to a 0 is in the host part of the IP address.