IP Address Summarization Question

[rfippinger]

Hello. I am trying to determine the summarized IP Address for the following range of addresses:

172.16.8.0/21

172.16.16.0/21

172.16.24.0/21

172.16.32.0/21

172.16.40.0/21

172.16.48.0/21

172.16.56.0/21

172.16.64.0/21

172.16.72.0/21

172.16.80.0/21

172.16.88.0/21

(Since the first two octets are the same for all of the addresses, I'll just ask about the 3rd octet). Looking at the 3rd octet for the first and last addresses (decimal 8 and 88) the bit pattern stops matching in the 8th bit position of the 3rd octet. Or in binary it looks like this:

8=00001000

88=01011000

Normally when summarizing you add all of the matching bits and adjust the subnet mask accordingly to arrive at the summarized address. Since there are no bits to add, would the summarized address be simply: 0. And is the subnet mask moved over to include this bit? To put it another way, would the address read:

172.16.0.0/17 (255.255.128.0)

Thank you for your time and feedback. Apologies if this is not terribly clear; I'm only on the first cup of coffee!

~Robert

[deilert]

Robert

This summarization covers these ranges

172.16.8.0/21 172.16.8.0 to 172.16.15.0

172.16.16.0/21 172.16.16.0 to 172.16.23.0

172.16.24.0/21 172.16.24.0 to 172.16.31.0

172.16.32.0/21 172.16..32.0 to 172.16.39.0

172.16.40.0/21 172.16.40.0 to 172.16.47.0

172.16.48.0/21 172.16.48.0 to 172.16.55.0

172.16.56.0/21 172.16.56.0 to 172.16.63.0

172.16.64.0/21 172.16.64.0 to 172.16.71.0

172.16.72.0/21 172.16.72.0 to 172.16.79.0

172.16.80.0/21 172.16.80.0 to 172.16.87.0

172.16.88.0/21 172.16.88.0 to 172.16.95.0

[rfippinger]

Wow, now that was a fast reply! Thank you for looking at my question. Judging by your response, I"m not summarizing correctly.

[mark_cairns]

Be careful with your summarization. By stating you have 172.16.0.0/17, you are advertising an IP range of 172.16.0.0 through 172.16.127.255, well out of the range you specified. Assuming you have 172.16.0.0 not in use as the first subnet, you can summarize 172.16.0.0/18 which includes 172.16.0.0 through 172.16.63.255. The last four subnets can be summarized on their own with 172.16.64.0/19 which advertises 172.16.64.0 through 172.16.95.255. Also check that your routes to null are correct if you are summarizing networks not in use to prevent routing loops.