# How to calculate Summary addr.?

Hi,

I'm trying to understand Summarization aka Route Aggregation. I don't understand what is it that I'm mistaking in this exercise:

"Find the summary address and mask of the following set of networks

a) 172. 148. 0. 0/13 through 172. 156. 0. 0/13"

MY ATTEMPT:

Clearly it's the 2nd octet that is changing, so the mask will be 255. ___.0.0

The number of networks are: (156 -148) +1= 9 nets

I need to cover at least 9 nets. Which block size will I use? Not 8 but 16.

16 means 4 bits on (starting from the left, from the most significant bit) i.e. 240.

So the mask should ultimately be:

255. 240. 0.0

and that means (8 +4)=12 bits on therefore it's going to be a /12

As to the summary addr., it's the 1st of the block, right?

Maybe 172. 148. 0. 0 /12 ?

However this answer is wrong.

CORRECT ANSWER

172. 144. 0. 0 /16

Where did I go wrong?