subnet mask

A number that identifies which part of an IP address corresponds to the network address and which corresponds to a host address on that network. In CIDR notation the mask is represented by a slash (/), followed by the number of bits as a decimal number (e.g., /24). In dotted-decimal notation the mask is represented by a dotted-decimal representation of the bit pattern for network and host address bits, where bits equal to 1 indicate that the corresponding bits in the IP address identify the network address, and bits equal to 0 identify the host address (e.g., 255.255.255.0, which is equivalent to /24). IP addresses with the same network identifier bits are on the same subnet.