help with route summarization please

newtechie1988

Hey there,
I am in my 2nd semester cisco networking. We are currently learning route summarization. I understand how to summarize a route but how do I do the reverse. for example when I am given the summarized route 172.168.8.0/21, how do I find out what the areas of networks it covers?
Could somebody please tell me and then give me some examples?
Thank you

jdfriesen

newtechie1988 wrote: »

Hey there,
I am in my 2nd semester cisco networking. We are currently learning route summarization. I understand how to summarize a route but how do I do the reverse. for example when I am given the summarized route 172.168.8.0/21, how do I find out what the areas of networks it covers?
Could somebody please tell me and then give me some examples?
Thank you

That's just a subnetting question. You just have to figure out the address range that 172.168.8.0/21 covers. /21 is equivalent to 255.255.224.0. That gives you a block size of 32 (256-224). In the case of your example, we actually have an error, because the first two subnets in the 172.168.0.0/21 range would be:

• 172.168.0.0
• 172.168.32.0

So, assuming that the original example was 172.168.32.0/21, then the covered addresses would be 172.168.32.0 - 172.168.63.255 (as 172.168.64.0 would be the next subnet).

Javi437

While u are on the right track I believe the subnet mask of /21 is 248 and therefore the increments are 16. Please correct me if I am wrong.

jdfriesen

Javi437 wrote: »

While u are on the right track I believe the subnet mask of /21 is 248 and therefore the increments are 16. Please correct me if I am wrong.

Reply, actually, we're both wrong! /21 is 248, but then the blocks are 8, so the original example is valid. Stupid mistake on my part.

That being the case, the address range referenced by 172.168.8.0/21 is then 172.168.8.0 - 172.168.15.255.

Sorry about that error.