Which of the following descriptions of wildcard masks is correct?

Question

Options

• AThe wildcard mask can be directly derived from the inversion of the binary subnet mask.
• BIn the wildcard mask, 0.0.0.0 represents all IP addresses.
• CThe wildcard mask must consist of consecutive "0" and "1" from left to right, and consecutive "1"s cannot be separated by "0".
• DIn the wildcard mask, setting the mask to "0" means that the corresponding bits in the IP address must match exactly.

• A
  12% (5)
• B
  2% (1)
• C
  5% (2)
• D
  81% (35)

Explanation

Option D correctly defines the core behavior of wildcard masks: a "0" bit means the corresponding IP address bit must match exactly, while a "1" bit means it's a "don't care" - any value is accepted. This is the opposite of subnet masks and is fundamental to how wildcard masks work in ACLs and routing protocols like OSPF.

Why the distractors are wrong:

• A is misleading - while you can invert a contiguous subnet mask to get a wildcard mask, wildcard masks are more general and can use non-contiguous bit patterns that no subnet mask can represent, so this isn't a universal derivation rule.
• B has it backwards - 0.0.0.0 means all bits must match exactly (matching only one specific host), not all addresses; 255.255.255.255 (all "don't care" bits) matches all addresses.
• C describes subnet mask rules, not wildcard masks - wildcard masks can freely mix 0s and 1s in any order (e.g., 0.255.0.255), which is precisely what makes them more powerful than subnet masks.

Memory tip: Think of the wildcard mask bit meaning as the opposite of what you'd expect - "0 = strict, 1 = wild" (the "1"s are the wild cards that can be anything). If you remember that, you can quickly eliminate B and D's confusion.

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