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In Figure 3-35, for example, route
summarization occurs at two levels.
?–  Router C summarizes two routing updates from networks 172.16.32.64/26 and
172.16.32.128/26 into a single update, 172.16.32.0/24.
?–  Router A receives three different routing updates but summarizes them into a single routing
update before propagating it to the corporate network.
172.16.168.0/24 =
172.16.169.0/24 =
172.16.170.0/24 =
172.16.171.0/24 =
172.16.172.0/24 =
172.16.173.0/24 =
172.16.174.0/24 =
172.16.175.0/24 =
Number of Common Bits = 21
Summary: 172.16.168.0/21
Noncommon
Bits = 11
10101100
172
172
172
172
172
172
172
00010000
16
16
16
16
16
16
16
00000000
0
0
0
0
0
0
0
10101
10101
10101
10101
10101
10101
10101
10101
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000
001
010
011
100
101
110
111
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Implementing Variable-Length Subnet Masks 131
Figure 3-35 Summarizing Addresses in a VLSM-Designed Network
Route summarization reduces memory use on routers and routing protocol network traf?¬?c.
Requirements for summarization to work correctly are as follows:
?–  Multiple IP addresses must share the same highest-order bits.


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