Router Summarisation

umarbhatti

Hi All

I have been trying to figure out the following router summarisation question for a work collegue who is also trying to study for his CCNA, but both of us cannot figure out how to proceed

"Which two subnetworks would be included in the summarized address of 172.30.80.0/20 (Choose Two)
172.31.17.4/30
172.31.51.16/30
172.31.64.0/18
172.31.80.0/22
172.31.92.0/22
172.31.192.0/18"

I do know that the route summarization will start after 172.31 and that the magic number is 16 but after that i am stumped on what to do

Regards

boredgamelad

I will assume you have a typo and meant "172.31.80.0/20":

255.255.240.0 = our interesting octet is the third.

256 - 240 = 16 is our magic number.

Since we're interested in the third octet, start with 0 there, and increment it by the magic number*. Our goal here is to find the range of the .80.0 subnet:

172.31.0.0
172.31.16.0
172.31.32.0
172.31.48.0
172.31.64.0
172.31.80.0
172.31.96.0

So 172.31.80.0/20 runs from 172.31.80.0 to 172.31.95.255. Now determine which subnets from that list fall completely within that range, and you have your answers.

*(Note: Since we were given 172.31.80.0/20 to begin with, we really didn't even need to do this step. We could have just started at 80 and added 16. But I like to check that the IP I was given is a valid subnet ID anyway).

hitmen

There is no magic number.
Just expand all the ip addresses to get a clearer picture.
Once you see the range of ip addresses. you will get your answer.

jmasterj206

Take a look at this video on youtube. It should clear up route summary for you.

How to find the Summary Route for the CCNA - YouTube