Which of the following masks, when used within a Class B network, would supply enough subnet bits to support 90 subnets? (Choose two)

(A) /24
(B) /21
(C) /19
(D) 255.255.255.252
(E) 255.255.240.0
(F) 255.255.224.0

A and D are the correct answers.

The masks in binary define a number of binary 1s, and the number of binary 1s defines the length of the prefix (network + subnet) part. With a Class B network, the network part is 16 bits. To support 90 subnets, the subnet part must be at least 7 bits long. Six subnet bits would supply only 26 = 64 subnets, while 7 subnet bits supply 27 = 128 subnets. The /24 answer supplies 8 subnet bits, and the 255.255.255.252 answer supplies 14 subnet bits.