N10-009 Question 115
Single answer1.7 Given a scenario, use appropriate IPv4 network addressing.A network administrator is configuring a new IPv4 subnet for a branch office. The office has 50 employee workstations, 6 network printers, 2 wireless access points, 1 router interface, and 1 server. The administrator wants to use the smallest private IPv4 subnet possible while allowing for a few additional devices in the future. Which subnet should the administrator choose?
- A
192.168.10.0/26
- B
192.168.10.0/25
- C
192.168.10.0/27
- D
192.168.10.0/28
Show answer and explanation
Correct answer: B
Explanation
To solve this scenario, first total the required devices: 50 workstations + 6 printers + 2 wireless access points + 1 router interface + 1 server = 60 hosts. The administrator also wants room for additional devices, so selecting a subnet with only 60 or 62 usable addresses would be too tight. A /26 provides 62 usable addresses, which leaves minimal room and does not reasonably satisfy the growth requirement. A /25 provides 126 usable addresses, making it the best choice among the options. This aligns with standard IPv4 subnetting practices described in common networking references and RFC-based addressing guidance, where private IPv4 ranges such as 192.168.0.0/16 are defined for internal use under RFC 1918. On Network+, candidates are expected to calculate usable hosts, account for network and broadcast addresses, and choose an appropriately sized subnet based on business needs rather than only current device counts.
- A. Incorrect.
Incorrect. A /26 subnet provides 64 total addresses and 62 usable host addresses. The branch office currently needs 60 usable addresses (50 workstations + 6 printers + 2 access points + 1 router interface + 1 server). While /26 technically fits the current requirement, it leaves only 2 usable addresses for growth, which does not meet the stated requirement of allowing for a few additional devices in the future. A candidate might choose this option because it appears to be the smallest subnet that works today, but the scenario explicitly requires extra capacity.
- B. Correct.
Correct. A /25 subnet provides 128 total addresses and 126 usable host addresses, which is enough for the current 60 devices and leaves substantial room for future growth. In IPv4 subnetting, the usable host formula is 2^h - 2, where h is the number of host bits. With 7 host bits in a /25, there are 126 usable addresses. This is the smallest option presented that satisfies both the current requirement and the need for additional devices in the future.
- C. Incorrect.
Incorrect. A /27 subnet provides 32 total addresses and 30 usable host addresses, which is far too small for the required 60 devices. This distractor reflects a common subnetting error in which someone underestimates host requirements or confuses total addresses with usable addresses.
- D. Incorrect.
Incorrect. A /28 subnet provides 16 total addresses and 14 usable host addresses, making it completely insufficient for this branch office. This option is plausible only if someone overlooks the number of devices that need addresses or misunderstands how subnet masks affect host capacity.