300-320 Question #332: Real Exam Question with Answer & Explanation

The correct answer is A: /20. To find the smallest summary prefix that covers at least 15 individual /24 networks, calculate how many /24 blocks each candidate prefix contains: /21 = 2^(24−21) = 2³ = 8 /24 networks (not enough), /20 = 2^(24−20) = 2⁴ = 16 /24 networks (covers 15, with 1 spare - sufficient), /1

Question

You need to design a network with a summary segment that supports up to 15 IP segments and all segments must be /24?

Options

A/20
B/21
C/18
D/19

Explanation

To find the smallest summary prefix that covers at least 15 individual /24 networks, calculate how many /24 blocks each candidate prefix contains: /21 = 2^(24−21) = 2³ = 8 /24 networks (not enough), /20 = 2^(24−20) = 2⁴ = 16 /24 networks (covers 15, with 1 spare - sufficient), /19 = 2^(24−19) = 2⁵ = 32 /24 networks (wasteful), /18 = 64 /24 networks (even more wasteful). The /20 summary is the most specific (smallest) prefix that can accommodate 15 /24 subnets while minimizing wasted address space. In practice, you need a block of 16 because address blocks must be powers of 2.

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