Hi, Below is the question with the explanation. I still dont understand how and why they subtracted 7 from 30 (30 – 7 = 23.). Can somebody help to explain please.

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

A. 10.10.0.0/18 subnetted with mask 255.255.255.252

B. 10.10.0.0/25 subnetted with mask 255.255.255.252

C. 10.10.0.0/24 subnetted with mask 255.255.255.252

D. 10.10.0.0/23 subnetted with mask 255.255.255.252

E. 10.10.0.0/16 subnetted with mask 255.255.255.252

Answer: D

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)