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Manual route summarization question

Posted on 2005-05-07
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172.16.17.0
172.16.18.0
172.16.19.0
172.16.20.0
172.16.21.0

The above networks are 172.16.0.0/21 manually summarized correct?
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Question by:dissolved

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Expert Comment

Not sure what you mean by manually summarized.  If you're asking if the listed networks are a subset of 172.16.0.0/21, the answer is no.  172.16.0.0/21 covers addresses from 172.16.0.0 to 172.16.7.255.
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Author Comment

what would the networks I listed be summarized?
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LVL 5

Expert Comment

172.16.16.0/21.

That network covers 172.16.16.0 to 172.16.23.255.
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Author Comment

I am doing something wrong in my formulas. Can you take a look at this and tell me if I am on the right track please?
Here goes:

172.16.17.0
172.16.18.0
172.16.19.0
172.16.20.0
172.16.21.0

The octet that differs above, is the third octet.  So we will place it in binary

17 is 00010001
18 is 00010010
19 is 00010011
20 is 00010100
21 is 00010101

They have 5 similar bits.

-Now I take 172.16 (16 bits) and add it to the "5" bits. This is how I get my "21."

What I do not understand, is how the network is 172.16.16.0  instead of 172.16.0.0  What am I doing wrong?
thanks
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LVL 5

Accepted Solution

You got /21.  Good.  That gives you the network bits that define the network that all the IP's noted are included in, namely the subnet mask.  Now you need to figure out the actual network that they're a part of.

We already know the first two octets of the network are 172.16.  From your binary calculations, you noted the networks have 5 similar bits: 00010xxx, which equals decimal 16.  That makes up your third octet, giving you 172.16.16.0.

Try doing it by hand and checking yourself with any of the many online IP subnet calculators.
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Author Comment

thanks for your patience and help
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