IP Address Allocation For Proper Summarization | zeroslash.io

IP Address Allocation For Proper Summarization

Submitted by zeroslash on Fri, 08/31/2018 - 10:39

I received a question from one of my webinar students about how to implement IP addressing when it comes to network design. This is a broad question but I wanted to give something that is useful. In fact, this practice is a must in any network.

One really important best practice is to ensure that your IP allocation across the network can be properly summarized. For example, contiguous IP address ranges should be used in a certain area or part of the network. A practical example would be like below.

You have 2 sites:

  • Site A
  • Site B

And you have a /21 network - 192.168.8.0/21 which can be broken down into 8 class C or /24 blocks.

192.168.8.0/24
192.168.9.0/24
192.168.10.0/24
192.168.11.0/24
192.168.12.0/24
192.168.13.0/24
192.168.14.0/24
192.168.15.0/24

When you allocate these you need to allocate a contiguous set of /24 blocks to a particular site.

For example:

  • Site A
192.168.8.0/24
192.168.9.0/24
192.168.10.0/24
192.168.11.0/24
  • Site B
192.168.12.0/24
192.168.13.0/24
192.168.14.0/24
192.168.15.0/24

 

With this allocation, you can summarize both sites as:

Site Summary prefix Specific prefixes
Site A 192.168.8.0/22 192.168.8.0/24
    192.168.9.0/24
    192.168.10.0/24
    192.168.11.0/24
Site B 192.168.12.0/22 192.168.12.0/24
    192.168.13.0/24
    192.168.14.0/24
    192.168.15.0/24

 

In a network topology, the routing updates will look like this -

IP Address Allocation With Proper Summarization
IP Address Allocation With Proper Summarization

 

The wrong way of doing this is like -

  • Site A
192.168.8.0/24
192.168.11.0/24
192.168.12.0/24
192.168.15.0/24
  • Site B
192.168.9.0/24
192.168.10.0/24
192.168.13.0/24
192.168.14.0/24

 

In this case, the routing updates will look like this -

IP Address Allocation Without Proper Summarization
IP Address Allocation Without Proper Summarization

 

It's impossible to summarize this kind of allocation. It's so non-contiguous there's no chance to summarize. When you allocate your IP addresses this way (I made it an extreme case on purpose), you'll easily increase the entries of your routing table. This issue might look simple at first glance, but if it goes unchecked, it might lead to other network design challenges such as convergence speed and network stability. It can impact convergence speed since the more routes you have, the more your routers have to process and propagate. It can also impact network stability since if you can't summarize, then all your specific prefixes are exposed to the rest of the network, even in parts of your network where these prefixes are not required to be seen (information hiding is a big part of network design). These 2 design concerns actually go hand in hand so IP address allocation and proper summarization are key to a better network.