Which of the following binary masks is equivalent to the 255.255.255.240 dotted-decimal notation (DDN)?

(A) 11111111.11111111.11111111.11111100
(B) 11111111.11111111.11111111.11110000
(C) 11111111.11111111.11111111.10000000
(D) 11111111.11111111.11111111.11111110

11111111.11111111.11111111.11110000 is the correct answer. There are only nine possible values in one octet of a subnet mask as shown in the table below. So start converting each octet one by one to get the final result.

1. First octet = 255 | First octet in binary = 11111111
2. Second octet = 255 | Second octet in binary = 11111111
3. Third octet = 255 | Third octet in binary = 11111111
4. Forth octet = 240 | Forth octet in binary = 11110000
Final result: 11111111.11111111.11111111.11110000

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