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Formulating a scalable IP addressing plan

A scalable IP addressing plan will support your customer's network as it grows. This tip explains how to use variable-length subnet masking and route summarization to create a scalable IP addressing plan.

A scalable IP addressing plan will support your customer's network as it grows. This tip, reposted courtesy of, explains how to use variable-length subnet masking and route summarization to create a scalable IP addressing plan.

Variable-length subnet masking

Variable-length subnet masking (VLSM) means implementing more than one mask on the same major class of a network. It allows for a more efficient use of IP address space both in terms of hosts and subnets. On a network

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that does not have an abundance of IP address space it can be essential. In order to implement different masks for the same major network it is required to have a routing protocol that supports VLSM. Such routing protocols are called classless routing protocols. They carry the mask information along with the route advertisements, therefore allowing for the support of more than one mask. Examples of classless routing protocols include OSPF, RIP version 2, Cisco's EIGRP, BGP and IS-IS.

Consider an example that employs VLSM. Assume that the Class B address is required to support a network that entails a total of 200 sites. The busiest LANs may support up to 100 hosts and there is a maximum projected total of 400 point-to-point WAN links. Hence there is a requirement for 600 subnets with a maximum of 100 hosts on any subnet. Even with a Class B address there is insufficient address space to meet this requirement without employing VLSM.

When planning a VLSM solution you should start with the shortest mask; in other words, plan the subnets that support the most hosts. This is typically the mask that will be used on most or all of the LAN segments. In the example, there are 200 LAN segments each supporting up to 100 hosts. While seven host bits or a /25 subnet mask would meet this requirement, it is probably neater in terms of administration to use a /24 mask. This is a luxury afforded simply because VLSM is being used in this case. The LAN segments can be numbered from to

Now it is time for the second stage of VLSM, which entails choosing from the available subnets and subnetting further. This is sometimes called "subnetting the subnets." It is important to remember that this can only be done with one or more subnets that have not already been used up. The range is free and could be subnetted with a /30 mask creating an additional 64 subnets in this range. Similarly, the 172.16.202.x/30 range produces 64 more subnets suitable for point-to-point links. Each range up to an including 172.16.207.x/30 could be used to provide enough subnet address space for 400 serials links. This means that the addressing requirements were met and there is still a considerable amount of address space free. Aim to use contiguous subnets where possible: Although it is not essential it certainly makes very good sense to choose a continuous range of addresses and apply a particular mask to them. Efficient allocation of IP addresses is not done merely for the sake of neatness; it is often essential for good network design.

Route summarization

Route summarization means summarizing a group of routes into a single route advertisement. The net result of route summarization and its most obvious benefit is a reduction is the size of routing tables on the network. This in turn reduces the latency associated with each router hop since the average speed for routing table lookup will be increased due to the reduced number of entries. The routing protocol overhead can also be significantly reduced since fewer routing entries are being advertised. This can become critical as the overall network (and hence the number of subnets) grows.

Apart from reducing routing table sizes, route summarization can also improve the stability of the network by containing the propagation of routing traffic after a network link goes down. If a router is only advertising a summary route to the next downstream router, then it will not advertise changes relating to specific subnets contained within the summarized range. For example, if a router only advertises the summary route to its adjacent neighbor then it will not update that neighbor if it detects a failure on the LAN segment. This principle can significantly reduce any unnecessary routing updates following a network topology change. Essentially this speeds up convergence resulting in a more stable network.

In order to implement route summarization that can be arbitrarily configured, a classless routing protocol is required, however that in itself is not enough. It is imperative to plan the IP addressing scheme such that non-conflicting summarization can be performed at strategic points in the network. These ranges are called contiguous address blocks. For example, a router that connects a group of branch offices to the head office could summarize all of the subnets used by those branch offices into a single route advertisement. If the subnets all fell within the range to then the range could be summarized as This is a contiguous range that also coincides with a perfect bit boundary thus ensuring that the address range can be summarized in a single statement. Clearly, to maximize the benefits of route summarization careful address planning is essential.

About the author
Cormac Long is the author of
IP Network Design and Cisco Internetworking and Troubleshooting.

This tip originally appeared on

This was last published in January 2007

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