CCNA Practice Exam Test Sample – Question 125

The network administrator is asked to configure 113 point-to-point links. Which IP addressing scheme defines the address range and subnet mask that meet the requirement and waste the fewest subnet and host addresses?

A. 10.10.0.0/16 subnetted with mask 255.255.255.252
B. 10.10.0.0/18 subnetted with mask 255.255.255.252
C. 10.10.1.0/24 subnetted with mask 255.255.255.252
D. 10.10.0.0/23 subnetted with mask 255.255.255.252
E. 10.10.1.0/25 subnetted with mask 255.255.255.252

Correct Answer: D
Section: IP addressing (IPv4 / IPv6)

Explanation:
We need 113 point-to-point links which equal to 113 sub-networks < 128 so we need to borrow 7 bits (because 2^7 = 128). The network used for point-to-point connection should be /30. So our initial network should be 30 – 7 = 23. So 10.10.0.0/23 is the correct answer. You can understand it more clearly when writing it in binary form: /23 = 1111 1111.1111 1110.0000 0000 /30 = 1111 1111.1111 1111.1111 1100 (borrow 7 bits)