Subnet Mask

A series of IP addresses can be organized into some categories, classes, or some groups. We specifically subdivide an available host address space into a group, which is called a sub-network. In the same subnet, computers can directly communicate each other. If the destination IP is not in the same subnet, it can be reached through a gateway or a router (network joint point). The subnet ID is denoted by some significant bits of the IP address. Subnet Mask is used to compute the subnet ID by AND with the IP addresses in this subnet.