Which of the following answers lists the prefix (CIDR) format equivalent of 255.255.254.0?

(A) /20
(B) /21
(C) /22
(D) /23

/23 is the correct answer.

Convert one octet at a time:
The first octet (255) convert to an 8-bit binary 11111111 total 8.
The second octet (255) convert to an 8-bit binary 11111111 total 8.
The third octet (254) convert to an 8-bit binary 11111110 total 7.
The fourth octet (0) convert to an 8-bit binary 00000000 total 0.
The equivalent of 255.255.254.0 is 11111111.11111111.11111110.00000000
The total number of binary 1s (which defines the prefix length) is 8 + 8 + 7 + 0 = /23.

Binary Mask Octet	Decimal Equivalent	Number of Binary 1s
00000000	0	0
10000000	128	1
11000000	192	2
11100000	224	3
11110000	240	4
11111000	248	5
11111100	252	6
11111110	254	7
11111111	255	8