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Variable-length subnet masking or VLSM can be defined as the capability to apply more than one subnet mask to a given class of addresses throughout a routed system. Although this is common practice in modern networks, there was a time when this was impossible because the routing protocols in use could not support it. Classful protocols such as RIPv1 do not include the subnet mask of advertised networks in their routing updates; therefore, they cannot possibly learn the existence of more than one mask length. Only classless routing protocols—EIGRP, OSPF, RIPv2, IS-IS, and BGP—include the subnet mask for the networks they advertise in their routing updates and thus publish a level of detail that makes VLSM possible.

The main push for VLSM came from the need to make networks the right size.

Subnetting logically creates the appropriately sized networks, but without the capability for routing protocols to advertise the existence (for example) of both a /26 and a /30 network within the same system. Prior to VLSM-capable routing protocols, the network in our example would have been confined to using only /26 masks throughout the system. The use of VLSM has two main advantages that are closely linked:

  • It makes network addressing more efficient.
  • It provides the capability to perform route summarization (discussed in the next section).

An illustration of the need for VLSM is shown in Figure 3.11.


FIGURE 3.11 Inefficient addressing without VLSM.

The diagram shows several branch offices using subnetted Class C (/26) addresses that provide each branch with 62 possible host IPs. The branches are connected to the central office via point-to-point WAN links. The ideal mask to use for such a link is /30 because it provides only two hosts, one for each end of the link. The problem arises when the routing protocols are configured: Prior to VLSM, the /30 networks could not be used because the /26 networks existed in the same system and the classful routing protocols could only advertise one mask per class of address. All networks, including the little /30 links, had to use the same mask of /26. This wastes 60 IP addresses on each WAN link.

With the implementation of VLSM-capable routing protocols, we can deploy a /30 mask on the point-to-point links, and the routing protocols can advertise them as /30s along with the /26s in the branches because the subnet mask for each network is included in the routing updates. Figure 3.12 illustrates the preferred, optimized addressing scheme that takes advantage of VLSM.


FIGURE 3.12 Optimized addressing using VLSM.

Note that using VLSM has allowed us to make the point-to-point link networks the ideal size (two hosts on each) using /30 masks. This has allowed us to use a single subnetted Class C network for all the addressing requirements in this scenario—and as you’ll see, it makes a perfect opportunity to summarize these routes. This is what is meant by “more efficient addressing”—in other words, making networks the right size without depleting the limited address space or limiting future growth.

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