In other words,
if you use any addresses from a subnet, that subnet cannot be further subnetted. In the example,
four subnet numbers are used on the LANs. Another unused subnet, 172.16.33.0/26, is further
subnetted for use on the WANs.
A
Routing Table
172.16.25.0/24
172.16.26.0/24
172.16.27.0/24
B
Routing Table
172.16.0.0/16
172.16.25.0/24
172.16.27.0/24
172.16.26.0/24
I can route to the
172.16.0.0/16 network.
Implementing Variable-Length Subnet Masks 129
The ?¬?gure 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.
Another advantage to using route summarization in a large, complex network is that it can isolate
topology changes from other routers. That is, if a speci?¬?c link in the 172.16.27.0/24 domain were
????¬‚apping,??? or going up and down rapidly, the summary route would not change. Therefore, no
router external to the domain would need to keep modifying its routing table due to this ?¬‚apping
activity. By summarizing addresses, you also reduce the amount of memory consumed by the
routing protocol for table entries.
Route summarization is most effective within a subnetted environment when the network
addresses are in contiguous blocks in powers of two.
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