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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 amounts 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 amount of host bits are chosen to express subnets. With variable length subnetting, the network ID being subnetted has already been subnetted.

Variable Length Subnetting Example

For example, given the class-based network ID of 135.41.0.0/16, a required configuration is to reserve half of the addresses for future use, create 15 subnets with up to 2,000 hosts, and 8 subnets with up to 250 hosts.

Reserve half of the addresses for future use

To reserve half of the addresses for future use, 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. The subnet 135.41.0.0/17 is chosen as the portion of the addresses which are reserved for future use.

Table 23 shows one subnet with up to 32,766 hosts.

Table 23 Reserving half the addresses for future use

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 24 illustrates 15 subnets with up to 2,000 hosts.

Table 24 15 Subnets with up to 2,046 hosts

8 Subnets with up to 250 Hosts

To achieve a requirement of 8 subnets with up to 250 hosts, a 3-bit subnetting of subnetted network ID of 135.41.248.0/21 is done, producing 8 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 25 illustrates 8 subnets with approximately 250 hosts.

Table 25 8 subnets with up to 254 hosts

The variable length subnetting of 135.41.0.0/16 is shown graphically in Figure 10.

Figure 10 Variable length subnetting of 135.41.0.0/16

Note In dynamic routing environments, variable length subnetting can only been 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 Short Path First (OSPF), and BGPv4 all support variable length subnetting.

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