Variable Length Subnetting

One of the original uses for subnetting was to subdivide a class-based network ID into a series of equal-sized subnets. For example, a 4-bit subnetting of a class B network ID produced 16 equal-sized subnets (using the all-ones and all-zeros subnets). However, subnetting is a general method of utilizing host bits to express subnets and does not require equal-sized subnets.

Subnets of different size can exist within a class-based network ID. This is well-suited to real-world environments, where networks of an organization contain different numbers of hosts, and different-sized subnets are needed to minimize the wasting of IP addresses. The creation and deployment of various-sized subnets of a network ID is known as variable length subnetting and uses variable length subnet masks (VLSM).

Variable length subnetting is a technique of allocating subnetted network IDs that use subnet masks of different sizes. However, all subnetted network IDs are unique and can be distinguished from each other by their corresponding subnet mask.

The mechanics of variable length subnetting are essentially that of performing subnetting on a previously subnetted network ID. When subnetting, the network ID bits are fixed and a certain number of host bits are chosen to express subnets. With variable length subnetting, the network ID being subnetted has already been subnetted.

For example, given the class-based network ID of 135.41.0.0/16, a required configuration is one subnet with up to 32,000 hosts, 15 subnets with up to 2,000 hosts, and eight subnets with up to 250 hosts.

One Subnet with up to 32,000 Hosts

To achieve a requirement of one subnet with approximately 32,000 hosts, a 1-bit subnetting of the class-based network ID of 135.41.0.0 is done, producing 2 subnets, 135.41.0.0/17 and 135.41.128.0/17. This subnetting allows up to 32,766 hosts per subnet. 135.41.0.0/17 is chosen as the network ID, which fulfills the requirement.

Table 1.23 shows one subnet with up to 32,766 hosts per subnet.

Table 1.23 One Subnet with up to 32,766 Hosts

Fifteen Subnets with up to 2,000 Hosts

To achieve a requirement of 15 subnets with approximately 2,000 hosts, a 4-bit subnetting of the subnetted network ID of 135.41.128.0/17 is done. This produces 16 subnets (135.41.128.0/21, 135.41.136.0/21 . . . 135.41.240.0/21, 135.41.248.0/21), allowing up to 2,046 hosts per subnet. The first 15 subnetted network IDs (135.41.128.0/21 to 135.41.240.0/21) are chosen as the network IDs, which fulfills the requirement.

Table 1.24 illustrates 15 subnets with up to 2,046 hosts per subnet.

Table 1.24 Fifteen Subnets with up to 2,046 Hosts

Eight Subnets with up to 250 Hosts

To achieve a requirement of eight subnets with up to 250 hosts, a 3-bit subnetting of subnetted network ID of 135.41.248.0/21 is done, producing eight subnets (135.41.248.0/24, 135.41.249.0/24 . . . 135.41.254.0/24, 135.41.255.0/24) and allowing up to 254 hosts per subnet. All 8 subnetted network IDs (135.41.248.0/24 to 135.41.255.0/24) are chosen as the network IDs, which fulfills the requirement.

Table 1.25 illustrates eight subnets with 254 hosts per subnet.

Table 1.25 Eight subnets with up to 254 Hosts

The variable length subnetting of 135.41.0.0/16 is shown graphically in Figure 1.10.

Figure 1.10 Variable Length Subnetting of 135.41.0.0/16

Note

In dynamic routing environments, variable length subnetting can only be deployed where the subnet mask is advertised along with the network ID. Routing Information Protocol (RIP) for IP version 1 does not support variable length subnetting. RIP for IP version 2, Open Shortest Path First (OSPF), and Border Gateway Protocol version 4 (BGPv4) all support variable length subnetting.