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Route Summarization (Cisco)

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by emekusman

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Hi everyone,
i seem to get stuck when I get to the subject of Route summarization. Look at this: summarizing 172.16.16.0 through 172.16.31.0. From the book it summarizes this to 172.16.0.0/20. Using methods of summarization outlined (bits from the left which are common to all addresses) I got the summarized address as 172.16.16.0/20. Could someone please explain this?

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by mshavrov

That's really easy:

Let's see what's common part between 172.16.16.0 and 172.16.31.0. To make it easier, will drop 172.16 part since it's noticable, and consider third octet:

16 = 0001 0000
31 = 0001 1111

So, first four bits stays the same and can be seen as "common" part.

First 2 octets (172.16) give us 16 bits of subnet mask, and we have extra 4 bits from the calculation above. Totally it's 20 bits, or sunetmask of 255.255.240.0.

If you have more questions, e-mail me.

Good luck,

by emekusman

Got your response. What i want to really clarify is the summarized address of 172.16.0.0/20. Why is it not 172.16.16.0/20?

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