Examine the table to determine the maximum number of bits (starting
from the left) that all of the addresses have in common (where they
stop lining up; we bolded them to make them easier for you to see).
The number of common bits is the subnet mask for the summarized
In our example, we can see from the table that all of the addresses have the
first 20 bits in common. The decimal equivalent of these first 20 bits is
172.16.96.0. So, we can write our new summarized address as 172.16.96.0/20.
If we were to later add a network 172.16.98.0, it would need to come off the
router summarizing this address space. If we didn't, it could cause problems.
Okay, this is confusing, we know. This is why we're going to give you three
Route Summarization Example 2
In this example, we will summarize 10.1.0.0 through 10.7.0.0. First, put
everything into binary, and then follow the bits, starting on the left and stop-
ping when the bits do not line up. Notice where we stopped boldfacing the
Now, create the network number using only the boldfaced bits. Do not
count the bits that are not in boldface. The second octet has no bits on (bits
in the bolded section), so we get this:
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