Short for subnetwork mask, a subnet mask is data used for bitwise operations on a network of IP addresses that has been divided into two or more groups. This process, known as subnetting, divides an IP network into blocks of logical addresses. Subnetting can improve security and help to balance overall network traffic.
A common example of a subnet mask for class C IP addresses is 255.255.255.0, the default subnet mask for many computers and network routers. When applied to subnet, a subnet mask shows the routing prefix.