Summarization address?

what is the summarization address for the networks
172.21.136.0 /24 and
172.21.143.0 /24 ?
I get 172.21.136.0 /21 but the Cisco press book says /20 im confused
they give alternatives :

A. 172.21.136.0 /21
B. 172.21.136.0 /20
C. 172.21.136.0 /22
D. 172.21.128.0 /2

Chapter 2 question 4 if someone wants to look in the book.

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Comments

Drakonblayde

Well, it wouldn't be the first time a Cisco Press book had a mistake in it...

Binarily,

10101100.00010101.10001 000.00000000
10101100.00010101.10001 111.00000000

So yeah, it's the first 21 bits that are common. The first 20 are as well, but in that case, the summary address would 172.21.128.0/20, not 172.21.136/20. They're both accurate, and the less bits used the less processing the router is going to have to do with the routes...

If it's saying Answer D is correct (assuming that the digit you left out is a 0 and not a 1) then yeah, it's the 'most correct' answer, since you're trying to cut router processing down with summarization, but if the number you left
out is a 1, then answer A is the most correct. Hell, if you *really* wanted to be **** you could cut that down to a /17 hehe

Paddington

The book says answer B.
Oh well im getting to be better then Cisco itself now time to take the test soon i guess

Drakonblayde

That's a typo then. I found several when I was going through the CNAP curriculum. But then, that's why we're here