Subnet Mask

A subnet is a particularly addressed part of a single Internet Protocol network. In the process of subnetting a computer network is divided into groups of computers which have the same designated IP address routing prefix. So we can say that if the IP address has two parts i.e. the network address and the host address then subnetting separates the IP addresses into the network and host addresses.

A subnet mask basically is a mask which tells you that to which subnet an IP address belongs to. We can understand it more clearly by the help of the following example, suppose there is an IP address 150.211.017.008. Let’s assume that this is a part of a Class B network, the first two numbers (150.215) represent the Class B network address while the second two numbers (017.009) identify a specific host on this network.

Subnetting an IP network is to split a big network into smaller multiple networks for reorganization and security purposes. Subnetting allows the network administrator to further divide the host part of the address into two or more subnets. In this case, a part of the host address is reserved to identify the particular subnet.

Applying a subnet mask to an IP address separates network address from host address. The network bits are represented by the 1's in the mask, and the host bits are represented by 0's. Performing a bitwise logical AND operation on the IP address with the subnet mask produces the network address.

For example, applying the Class C subnet mask to our IP address 216.3.128.12 produces the following network address:

IP:   1101 1000. 0000 0011. 1000 0000. 0000 1100 (216.003.128.012)
Mask: 1111. 1111. 1111 1111. 0000                           (255.255.255.000)

      ---------------------------------------------
      1101 1000. 0000 0011. 1000 0000. 0000              (216.003.128.000)