Discontiguous
networks and summarization are discussed later in this chapter.
130 Chapter 3: Medium-Sized Routed Network Construction
?– 172.16.173.0/24
?– 172.16.174.0/24
?– 172.16.175.0/24
To determine the summary route, the router determines the number of highest-order bits that
match in all the addresses. By converting the IP addresses to the binary format, as shown in Figure
3-34, you can determine the number of common bits shared among the IP addresses.
Figure 3-34 Summarizing Within an Octet
In Figure 3-34, the ?¬?rst 21 bits are in common among the IP addresses. Therefore, the best
summary route is 172.16.168.0/21. You can summarize addresses when the number of addresses
is a power of two. If the number of addresses is not a power of two, you can divide the addresses
into groups and summarize the groups separately.
To allow the router to aggregate the highest 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.
A VLSM design allows for maximum use of IP addresses, as well as more ef?¬?cient routing update
communication when using hierarchical IP addressing.
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