| 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.
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