Which address would successfully summarize only the networks seen?

A. 192.168.0.0/24

B. 192.168.8.0/20

C. 192.168.8.0/21

D. 192.168.12.0/20

E. 192.168.16.0/21

F. These networks cannot be summarized.




 

Suggested Answer: C

Let's suppose it is a /20 then we would have addresses from 192.168.0.1 to 192.168.15.255
Now let's suppose it is a /21 then we would have addresses from 192.168.8.1 to 192.168.15.255
So both summaries encompass the networks we want to summarize but the second one is the most restrictive one as it only encompasses the networks we were asked to summarize and not others so it is the correct summary.
In fact just count the number if subnets which is 8 and find the exponent of 2 which is 8, that gives you 3 and to find out the summary mask just do /24 - 3 = /21.
If you make it an binary you'll find out the answer too:
the interesting octet is 3rd one so let's convert in binary.
8    00001 000
9    00001 001
10  00001 010
11  00001 011
12  00001 100
13  00001 101
14  00001 110
15  00001 111
I've bolded the bits in common there are 5 so 16+5=21 which is mask and address is 192.168.8.0

This question is in 300-101 Cisco Implementing Cisco IP Routing (ROUTE) Exam
For getting Cisco Certified Network Professional (CCNP) Routing and Switching Certificate




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