Network Sales and Services Handbook (Cisco Press Networking Technology)

Technical Note: Subnetting and Subnet Masks

Subnets are portions of a network sharing a common address component. On TCP/IP networks, subnets are defined as all devices whose IP addresses have the same prefix. For example, all devices with an IP address starting with 100.100.100 would be part of the same subnet. Dividing a network into subnets is useful for security and performance reasons. IP networks are divided using a subnet mask.

Subnet masks are used to determine the subnet to which an IP address belongs. An IP address has two components: the network address and the host address. For example, consider the IP address 150.215.017.009. Assuming that this is part of a Class B network, the first two numbers (150.215) represent the Class B network address, and the second two numbers (017.009) identify a particular host on the 150.215 network.

Subnetting enables network administrators to divide the host part of the address into two or more subnets. In this case, a part of the host address identifies the particular subnet.

Subnetting often is easier to see when the IP address is shown in binary format. This is the binary format for the 150.215.017.009 address:

10010110.11010111.00010001.00001001

The Class B network part is as follows:

10010110.11010111

The host address is shown here:

00010001.00001001

If this network is divided into 14 subnets, the first 4 bits of the host address (0001) identify the subnet.

The subnet mask is the network address plus bits reserved for identifying the subnetwork. In this case, the subnet mask is 11111111.11111111.11110000.00000000. It's called a mask because it identifies the subnet to which an IP address belongs by performing a Boolean algebraic AND operation on the mask and the IP address. The result is the following subnetwork address: