Case Study VLSMs

In 1987, RFC 1009 was published with the purpose of specifying how a subnetted network could use more than one subnet mask. As discussed earlier in this chapter, when an IP network is assigned more than one subnet mask, it is considered a network with variable-length subnet masks because the subnet masks (prefixes) have varying lengths.

If you recall, the use of VLSM brings benefits to a network and routing that allow for increased routing optimization in the form of a smaller and more concise routing table, known as route aggregation, as well as more efficient use of an organization's assigned IP address space.

A /16 network with a /22 extended-network prefix permits 64 subnets (26), each of which supports a maximum of 1022 hosts (210x2). See Figure 1-17.

Figure 1-17 Using a VLSM Extended-Network Prefix

- Network Prefix -


Subnet " bits ~


Host bits -


<-VLSM Network Prefix->

This is fine if the organization wants to deploy a number of large subnets, but what about the occasional small subnet that contains only 20 or 30 hosts? Because a subnetted network could have only a single mask, the network administrator was still required to assign the 20 or 30 hosts to a subnet with a 22-bit prefix. This assignment would waste approximately 1000 IP host addresses for each small subnet deployed. Limiting the association of a network number with a single mask did not encourage the flexible and efficient use of an organization's address space.

One solution to this problem was to allow a subnetted network to alter its subnet mask through the use of VLSM. Assume that in Figure 1-17, the network administrator is allowed to configure the network with a /26 extended-network prefix. See Figure 1-18.

42 Chapter 1: Networking and Routing Fundamentals

Figure 1-18 Considerably Extending the Network Prefix with VLSM

- Network Prefix -


Subnet " Bits ~


Real Long VLSM Network Prefix

A /16 network address with a /26 extended-network prefix permits 1024 subnets (210), each of which supports a maximum of 62 hosts (26x2). The /26 prefix would be ideal for small subnets with less than 60 hosts, while the /22 prefix is well suited for larger subnets containing up to 1000 hosts. This is VLSM in action, that is, several different masks in use within a network. The next section takes a look at how you can take an IP address range and subnet it into many different sizes so that you can meet the needs of every part of your organization.

Route Aggregation

VLSM also allows the recursive division of an organization's address space so that it can be reassembled and aggregated to reduce the amount of routing information at the top level. Conceptually, a network number is first divided into subnets, some of the subnets are further divided into sub-subnets, and those are further divided as well. This allows the detailed structure of routing information for one subnet group to be hidden from routers in another subnet group.

In Figure 1-19, the network is first configured with a /16 extended-network prefix. The subnet is then configured with a /24 extended-network prefix, and the subnet is configured with a /19 extended-network prefix. Note that the recursive process does not require the same extended-network prefix to be assigned at each level of the recursion. Also, the recursive subdivision of the organization's address space can be carried out as far as the network administrator needs to take it.

Figure 1-19 Dividing a Network Prefix with VLSM

M H M k

Figure 1-20 illustrates how a planned and thoughtful allocation of VLSM can reduce the size of an organization's routing tables and conserve IP address space. Notice how Routers F and G are able to summarize the six subnets behind them into a single advertisement ( and, respectively) and how Router B ( is able to aggregate all the subnets behind it into a single advertisement. Likewise, Router C is able to summarize the six subnets behind it into a single advertisement ( Finally, the subnet structure is not visible outside of the organization, because through the use of VLSM and aggregation, Router A injects a single route into the global Internet's routing table— (or 10/8).

44 Chapter 1: Networking and Routing Fundamentals

Figure 1-20 VLSM in Action

Router D

Router E

Router A

Router A

Router F

Router D

Router E

Router F

Router G

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