Route Summarization

a quick question because i dont have the equipment to test what i am thinking.

okay in route summarization if you have a list of networks say 172.16.x.0 where x is less than 128 but more than 64 can the subnet mask of the summarized route be 255.255.64.0. it doesnt sound right but the possibility keeps running hrough my mind.

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Comments

r_durant

I don't think a "subnet mask" can ever be .64.0, they can be 128, 192, 224, 248, 252, 254 or 255

But are you referring to the subnet mask or the summarized subnet number? In that case, it can be 172.16.64.0 with a 255.255.128.0 and that would cover subnets between 64.0 to 127.255

bighornsheep

List of networks 172.16.x.0 where x is > 64 & < 128, you're trying to summarize:
172.16.64.1 - 172.16.127.255

The summary route would be 172.16.64.0 /18 which includes IP addresses:
172.16.64.0
|
172.16.127.255

dtlokee

Kinda the easy way I remember how to do this comes down to the incremental values of each subnet mask. If you have your powers of 2 (1,2,4,8,16,32,64,128,256) just find the one that will best fit the addresses you're summarizing then subtract from 256 and that will give you your mask. This only works when the range fits into one of the multiple values but for CCNA that is what is expected.

for example:

192.168.8.0-192.168.15.0
of the powers of 2 8 will be the best match (15-8 = 7) so take 256 - 8 and you get 248. the new mask will be 255.255.248.0.

dunno if it helps but that is how I do them in my head. Also in this example I made a range that works with an increment of 8, if the range was 192.168.14.0 - 192.168.21.0 it wouldn't work with the multiple of 8

r_durant

bighornsheep wrote:

The summary route would be 172.16.64.0 /18 which includes IP addresses:
172.16.64.0
|
172.16.127.255

Thanks for highlighting an error in my post bighorn...should have been with a 255.255.192.0 or as u said a /18