Which of the following prefix-lists is the most specific match for prefixes 192.168.64.1/20 and 192.168.32.5/19?

Question

Options

• APrefix 192.168.0.0/17 longer.
• BPrefix 192.168.0.0/19 longer.
• CPrefix 192.168.0.0/10 longer.
• DPrefix 192.168.0.0/18 longer.

• A
  78% (14)
• B
  6% (1)
• C
  6% (1)
• D
  11% (2)

Explanation

Option A (192.168.0.0/17) is the most specific supernet that contains both target prefixes. A /17 mask fixes the first 17 bits, covering 192.168.0.0–192.168.127.255 (third octet 0–127). Since 192.168.64.x (64 < 128) and 192.168.32.x (32 < 128) both fall inside this range, the longer keyword then matches any more-specific routes (like /19 and /20) within it - making this the tightest possible supernet umbrella for both.

Why the distractors fail:

• B (/19): Covers only 192.168.0.0–192.168.31.255; neither 192.168.32.x nor 192.168.64.x falls in that range (32 and 64 are both ≥ 32, outside the /19 block starting at 0).
• C (/10): Does technically contain both prefixes, but /10 is far less specific than /17 - it spans a massive 192.128.0.0–192.191.255.255 range, so it's the least specific option, not the most.
• D (/18): Covers 192.168.0.0–192.168.63.255; 192.168.32.x fits, but 192.168.64.x does not (64 > 63).

Memory tip: Convert the third octet to binary and count shared leading bits across all target prefixes - that bit count is your most-specific common prefix length. Here, 64 = 01000000 and 32 = 00100000 share only the first bit (0), placing them both under a /17 boundary.

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