2.5 Route Summarization Overview
2.5.2 Summarizing within an octet
The main figure illustrates a summary route based on a full octet --- 172.16.25.0/24, 172.16.26.0/24, and 172.16.27.0/24 could be summarized into 172.16.0.0/16. What if a router received updates for the following routes? How would the routes be summarized?

Consider the following list of network addresses:

  • 172.108.168.0
  • 172.108.169.0
  • 172.108.170.0
  • 172.108.171.0
  • 172.108.172.0
  • 172.108.173.0

To determine the summary route, the router looks for the most highest-order number of bits that match. Referring to the list of IP addresses in the main figure, the best summary route is 172.108.168.0/21. 
NOTE: With this summary address includes all of the following addresses: 172.108.168.0 thru 172.108.175.0. Even though172.108.174.0 and 172.108.175.0 were not planned for, the summary address includes these two addresses as well. This means that the next block of available addresses outside of this summary address begins with 172.108.176.0.

To allow the router to aggregate the most number of IP addresses into a single route summary, your IP addressing plan should be hierarchical in nature. This approach is particularly important when using VLSMs, as illustrated in the next section.

In addition, you can summarize when the count is a power of two. The starting octet must be a multiple of the count. For example, you can summarize 8 bits starting with a multiple of 8, or 16 bits starting with a multiple of 16.