200-301 Question #854: Real Exam Question with Answer & Explanation

Question

You have the Class B network 172.16.0.0/16 and want to create 16 subnets. What will the new subnet mask be for the 16 subnets?

Options

• A/18
• B/19
• C/21
• D/17
• E/22
• F/20

Explanation

To create 16 subnets from a /16 network, four host bits must be borrowed, resulting in a new subnet mask of /20.

Common mistakes.

• A. A /18 mask would provide 2^(18-16) = 2^2 = 4 subnets, which is insufficient.
• B. A /19 mask would provide 2^(19-16) = 2^3 = 8 subnets, which is insufficient.
• C. A /21 mask would provide 2^(21-16) = 2^5 = 32 subnets, which is more than needed.
• D. A /17 mask would provide 2^(17-16) = 2^1 = 2 subnets, which is insufficient.
• E. A /22 mask would provide 2^(22-16) = 2^6 = 64 subnets, which is more than needed.

Concept tested. Subnetting calculations

Reference. https://learn.microsoft.com/en-us/training/modules/network-fundamentals/8-ip-addressing-subnetting

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