Refer to the exhibit. Which mask is correct to use for the WAN link between the routers that will
provide connectivity while wasting the least amount of addresses?

A.
/23

B.
/24

C.
/25

D.
/30

Explanation:
This question is to examine the IP addressing scheme.
In the serial port network, only two IP addresses can be assigned. In order to save IP addresses,
we often use the /30 subnet mask.
So the correct answer is D.