Which of the following correctly pairs the dotted decimal subnet mask with the correct number of binary bits that represent the subnet mask?

Question

Options

• A255.255.255.192 and /25
• B255.255.255.248 and /28
• C255.255.255.224 and /26
• D255.255.255.248 and /27
• E255.255.255.240 and /28
• F255.255.255.240 and /16

• B
  2% (1)
• C
  4% (2)
• E
  82% (37)
• F
  11% (5)

Explanation

Option E is correct because 255.255.255.240 in binary is 11111111.11111111.11111111.11110000 - counting all the consecutive 1-bits gives exactly 28, making /28 the accurate CIDR equivalent.

Why the distractors are wrong:

• A pairs 255.255.255.192 (11000000) with /25 - that last octet has two 1-bits (24+2=26), so it should be /26.
• B pairs 255.255.255.248 (11111000) with /28 - five 1-bits in the last octet (24+5=29) makes it /29.
• C pairs 255.255.255.224 (11100000) with /26 - three 1-bits (24+3=27) makes it /27.
• D pairs 255.255.255.248 with /27 - still /29 as explained above, just a different wrong number.
• F pairs 255.255.255.240 with /16 - the mask is correct but /16 would be 255.255.0.0, nowhere close.

Memory tip: Convert the last octet to binary and count all 1s across all four octets total. A fast shortcut: memorize the "magic eight" last-octet values - 128(/1), 192(/2), 224(/3), 240(/4), 248(/5), 252(/6), 254(/7), 255(/8) - and add 24 (for the first three full octets) to get the CIDR prefix.

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