CISCO CCNA Exam – Q138

Refer to the exhibit.
CISCO CCNA Exam – Q138

In this VLSM addressing scheme, what summary address would be sent from router A?

A. 172.16.0.0 /16
B. 172.16.0.0 /20
C. 172.16.0.0 /24
D. 172.32.0.0 /16
E. 172.32.0.0 /17
F. 172.64.0.0 /16

Correct Answer: A

Explanation:
Router A receives 3 subnets: 172.16.64.0/18, 172.16.32.0/24 and 172.16.128.0/18.
All these 3 subnets have the same form of 172.16.x.x so our summarized subnet must be also in that form.
The smallest subnet mask of these 3 subnets is /18 so our summarized subnet must also have its subnet mask equal or smaller than /18.
Only answer A has these 2 conditions .