12/30
2 Hosts
WAN Subnets
Derived from
172.16.33.0/26
LAN Subnets
Derived from
172.16.32.0/20
Entire Region Subnet
172.16.32.0/20
128 Chapter 3: Medium-Sized Routed Network Construction
To calculate the subnet addresses used on the WAN links, further subnet one of the unused /26
subnets. In this example, 172.16.33.0/26 is further subnetted with a pre?¬?x of /30. This provides
four more subnet bits, and therefore 16 (24) subnets for the WANs.
Route Summarization with VLSM
In large internetworks, hundreds or even thousands of network addresses can exist. In these
environments, it is often not desirable for routers to maintain many routes in their routing table.
Route summarization, also called route aggregation or supernetting, can reduce the number of
routes that a router must maintain by representing a series of network numbers in a single
summary address. This section describes and provides examples of route summarization,
including implementation considerations.
Figure 3-33 shows that Router A can either send three routing update entries or summarize the
addresses into a single network number.
Figure 3-33 VLSM Route Summarization
NOTE Remember that only subnets that are unused can be further subnetted.
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