Which Subnet Mask Provides Six Usable IP Addresses?

N10-009 By islever Editorial Last updated

Short answer

D 255.255.255.240

Question

A customer needs six usable IP addresses. Which of the following best meets this requirement?
  1. 255.255.255.128
  2. 255.255.255.192
  3. 255.255.255.224
  4. 255.255.255.240 Correct Answer

Key Takeaways

This question tests the ability to calculate usable host counts and choose the smallest subnet that satisfies a requirement; the common trap is miscalculating the host count for /27, which actually provides 30 usable addresses, not 6.

For a customer needing six usable IP addresses, the best subnet mask among the options is 255.255.255.240 (/28), which yields 14 usable hosts. Community consensus supports option D as the most efficient choice, noting that an exact match would require a /29 subnet.

Common mistake: The most common wrong answer is C (255.255.255.224), selected by about 23% of voters, often because they mistakenly believe it provides exactly 6 usable IPs. In reality, /27 provides 30 usable addresses and wastes more addresses compared to /28.

Expert Analysis

Why the Answer Is Correct

Option D, 255.255.255.240 (/28), has 4 host bits, yielding 2^4 - 2 = 14 usable IP addresses. This is the smallest subnet listed that still meets the requirement of 6 usable addresses, minimizing wasted address space. As comment [1] notes, the ideal mask would be /29 (255.255.255.248) for exactly 6 usable addresses, but /28 is the next best option among those provided.

Why the Other Options Are Wrong

Options A, B, and C all provide far more than needed: /25 gives 126 usable, /26 gives 62 usable, and /27 gives 30 usable. Comment [3] incorrectly claims /27 provides 6 usable addresses, but that is a miscalculation—/27 has 5 host bits, not 3. The question asks for the best fit, so larger subnets that waste more addresses are not the correct choice.

Community Comment Notes

The consensus in the comments is clearly option D, with 77% of voters selecting it. Comment [5] emphasizes efficiency, pointing out that /28 provides enough addresses without wasting as many as /27. Comment [2] correctly calculates the /28 host count as 14. The minority who chose C appear to have misunderstood host-bit calculations or ignored the goal of minimizing wasted addresses.

Official Reference

https://www.comptia.org/certifications/network

This link opens the official vendor documentation in a new tab.

Exam Strategy

On subnetting questions, always determine the required number of usable hosts, then find the smallest subnet mask that provides at least that many usable addresses using the formula (2^n - 2). Remember that an exact option may not exist, so choose the one with the least waste while still covering the requirement.

Community Votes

D
77%
C
23%

77% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 8 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 8

  1. kinkistyle 👍 4 Answered D
    The ideal mask would be 255.255.255.248 which would give us exactly the right amount of usable addresses at 6, but the next best is D. with 14 usable address
  2. Powerserg28 👍 3 Answered D
    255.255.255.240 = 11111111.1111111.1111111.11110000 Number of networks = 2^n; n = on bits (1); 2^4 = 16 Number of host = (2^n) - 2; n = off bits (0); (2^4) - 2 = 14
  3. BobbyCruz111 👍 2 Answered C
    The subnet mask 255.255.255.224 (option C) provides 6 usable IP addresses, which is exactly what the customer needs. 255.255.255.240 (option D) provides 14 usable IPs, which is more than needed. So, C. 255.255.255.224 is the best answer for 6 usable IP addresses.
  4. ba10f26 👍 1 Answered D
    Correct Answer: D 255.255.255.240 provides 14 usable IP addresses, which is more than enough for the 6 usable IPs the customer needs. This option is more efficient compared to 255.255.255.224, which provides 30 usable IPs and would waste a larger number of addresses.
  5. chupapi_001 👍 1 Answered D
    Since the customer needs 6 usable IP addresses, we should choose the smallest subnet that can accommodate this requirement while minimizing wasted addresses. 255.255.255.240 provides 14 usable addresses, which is the closest match to the requirement of 6 addresses.
Show 3 more comments
  1. S1vu 👍 1 Answered D
    255.255.255.240 (Option D) is the most efficient choice, as it provides 14 usable addresses, which meets the requirement without wasting too many additional addresses.
  2. Philco 👍 1 Answered C
    Since you need 6 IP addresses, the closest power of 2 that can accommodate that is 2^3 (which equals 8), meaning you need 3 host bits
  3. S1vu 👍 1
    Correct answer is D (14 addresses), It is the closest to 6 usable IP addresses without wasting any additional addresses

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