Which of the following prefix-lists is the most specific match for prefix 172.31.2.1/24?

Question

Options

• APrefix 172.16.0.0/12 longer.
• BPrefix 172.31.0.0/22 longer.
• CPrefix 172.31.0.0/23 longer.
• DPrefix 172.31.0.0/24 exact.

• A
  4% (1)
• B
  87% (20)
• C
  9% (2)

Explanation

Option B matches 172.31.2.0/24 because the prefix 172.31.2.0 falls within the 172.31.0.0/22 block (which spans 172.31.0.0–172.31.3.255), and /24 is longer than /22, satisfying the longer keyword. Among all matching options, /22 is the narrowest base prefix, making B the most specific match.

Why the distractors fail:

• A (172.16.0.0/12 longer) also matches - 172.31.2.0/24 is within the /12 block - but /12 is a far broader range than /22, so it's a less specific match.
• C (172.31.0.0/23 longer) does not match: the /23 block covers only 172.31.0.0–172.31.1.255, and 172.31.2.0 falls outside that range.
• D (172.31.0.0/24 exact) does not match: it requires an exact match, and 172.31.0.0/24 is a different network than 172.31.2.0/24.

Memory tip: Think of prefix-list matching as a two-step filter - first, does the address fall inside the base prefix's range? Second, does the prefix length satisfy the condition (longer, ge, le, exact)? "More specific" means the narrowest base range that still contains your prefix - a /22 fence around your address beats a /12 fence.

No community discussion yet for this question.