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Chapter 3
IP Addressing
Notice where the bits stop lining up (in boldface). Count only the bits that
are on (ones) to get the network address:
172.16.0.0
Now, create the summary mask by counting all the bits that are in bold-
face up to the point where they stop lining up. We have eight bits in the first
octet, eight bits in the second octet, and four bits in the third octet. That is
a /20 or
255.255.240.0
Boy, that sure seems like a pain in the pencil, huh? Try this shortcut. Take
the first number and the very last number, and put them into binary:
Can you see that we actually came up with the same numbers? It is a lot
easier than writing out possibly dozens of addresses. Let's do another exam-
ple, but let's use our shortcut.
Route Summarization Example 4
In this example, we will show you how to summarize 192.168.32.0 through
192.168.63.0. By using only the first network number and the last, we'll save
a lot of time and come up with the same network address and subnet mask:
First number: 192.168.32.0 =
11000000.10101000.00100000.00000000
Last number: 192.168.63.0 =
11000000.10101000.00111111.00000000
Network address: 192.168.32.0
Subnet mask: 255.255.224.0
172.16.16.0
10101100.0001000.00010000.00000000
172.16.31.0
10101100.0001000.00011111.00000000
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