Which of the following answers lists the dotted decimal notation (DDN) equivalent of /18?

(A) 255.255.255.192
(B) 255.255.224.0
(C) 255.255.128.0
(D) 255.255.192.0

255.255.192.0 is the correct answer.

/18 is the equivalent of the mask that in binary has 18 binary 1s. To convert that to DDN format, write down all the binary 1s (18 in this case), followed by binary 0s for the remainder of the 32-bit mask. Then take 8 bits at a time and convert from binary to decimal.
Using the /18 mask in this question, the binary mask is 11111111.11111111.11000000.00000000. Each of the first two octets is all binary 1s, so each converts to 255.
The third octet, 11000000, converts to 252, for a DDN mask of 255.255.192.0.

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