Planning Variable Length Subnet Masks (VLSM)

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Variable length subnet masks (VLSMs) allow you to use different prefix lengths at different locations so that subnets of different sizes can coexist on the same network. Instead of using one subnet mask throughout the network, you apply several masks to the same address space, producing subnets of different sizes. For example, given the Class B network ID of 131.107.0.0, you can configure one subnet with as many as 32,766 hosts, 15 subnets with as many as 2,046 hosts, and 8 subnets with as many as 254 hosts.

Tip

• When using VLSM, do not accidentally overlap blocks of addresses. If possible, start with equal-size subnets and then subdivide them.

VLSM also can be used when a point-to-point WAN link connects two routers. One way to handle such a WAN link is to create a small subnet consisting of only two addresses. Without VLSM, you might divide a Class C network ID into an equal number of two-address subnets. If only one WAN link is in use, all the subnets but one serve no purpose, wasting 252 addresses.

Alternatively, you can divide the Class C network into 16 workgroup subnets of 14 nodes each by using a prefix length of 28 bits (or, in subnet mask terms, 255.255.255.240). By using VLSM, you can then subdivide one of those 16 subnets into 8 smaller subnets, each supporting only 2 nodes. You can use one of the 8 subnets for your existing WAN link and reserve the remaining 7 subnets for similar links that you might need in the future. To accomplish this act of sub-subnetting by using VLSM, use a prefix length of 30 bits (or, in subnet mask terms, 255.255.255.252).

Figure 1.8 shows variable length subnetting for two-host WAN subnets.

Figure 1.8   Variable Length Subnetting of 131.107.106.0

If your network includes numerous WAN links, each with its own subnet, this approach can require significant administrative overhead. If you do not use route summarization, each subnet requires another entry in the routing table, increasing the overhead of the routing process.

Some routers support unnumbered connections; a link with unnumbered connections does not require its own subnet.