What Is the Correct Subnet Mask for an ISP-Assigned /28 Range?

N10-009 By islever Editorial Last updated Sep 2, 2026

Short answer

A Change the subnet mask to .240.

Question

An ISP provided a company with an IP range of 98.174.235.142/28. A network technician configured a router with a subnet mask of 255.255.255.224 and default gateway of 98.174.235.129. After the configuration was set up. the company is unable to connect to the ISP. Which of the following would resolve this issue?

Change the subnet mask to .240. Correct Answer
Change the Ethernet cable from the external interface to the modem.
Change the CIDR to /29.
Change the static IP to 98.174.235.128/28.

Key Takeaways

The question tests whether you understand slash notation, subnet masks, and usable host addresses—the key trap is confusing the network address with an assignable host IP.

Learn why the correct fix for an ISP /28 address configuration is changing the router's subnet mask from 255.255.255.224 (/27) to 255.255.255.240 (/28). Community experts agree that option A resolves the connectivity failure while options like changing the IP to .128 are invalid.

Common mistake: The most common wrong answer is D, choosing to change the static IP to 98.174.235.128/28. Many test takers mistakenly believe this aligns with the /28 block, but .128 is the network address and cannot be assigned to a host; the original .142 is a valid usable address in the .128/28 range.

Expert Analysis

Why the Answer Is Correct

Option A fixes the router's subnet mask to 255.255.255.240, which is the dotted-decimal form of a /28 prefix. The ISP assigned 98.174.235.142/28, so the router must be configured with a /28 mask to match the assigned subnet boundary. The default gateway 98.174.235.129 is the first usable host in the .128/28 network, and .142 is the last usable host, so both are valid. Using /27 (255.255.255.224) makes the local subnet too large and can cause routing or reachability problems. Thus, changing the mask to .240 directly resolves the mismatch.

Why the Other Options Are Wrong

Option B is unrelated because replacing the Ethernet cable does not affect IP addressing or subnetting. Option C, changing the CIDR to /29, would shrink the subnet to only 8 addresses and does not match the ISP-provided /28 range. Option D, changing the static IP to 98.174.235.128/28, is invalid because .128 is the network address, not a usable host address. The current IP .142 is within the /28 block and does not need to be changed; the only missing piece is the correct subnet mask.

Community Comment Notes

Comment [1] correctly summarizes that you simply need to change the subnet mask. Comment [2] reinforces that .240 equals /28, which aligns with the ISP-provided range. Some users who voted for D may have confused the network address with a usable IP, but the network address .128 cannot be configured as a host. The community consensus, supported by subnetting fundamentals, is that option A is the correct fix.

Official Reference

These links open the official vendor documentation in a new tab.

Exam Strategy

On exam day, when you see an IP with a prefix like /28, immediately convert it to its dotted-decimal mask: /28 = 255.255.255.240. Check that both the assigned IP and the gateway fall inside the same subnet, and never choose a network or broadcast address as a host IP.

Community Votes

83%

17%

83% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 3 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 3

TDWA2k 👍 4 Answered A

You need to change the subnet mask
93831b0 👍 1 Answered D

Option A: Changing the subnet mask to .240 (which would be /23 in CIDR notation) might not resolve the issue as it alters the subnet range and could cause misaddressing of devices within the current network. Option B: Altering the Ethernet cable configuration from an external interface to a modem is unrelated to IP routing or subnetting, so this option does not address the root cause of the problem. Option C: Modifying the CIDR to /29 would move beyond the intended /28 subnets, which might be unnecessary and could lead to issues with existing networks. Option D: Changing the static IP from 98.174.235.142/28 to 98.174.235.128/28 is the most likely solution. This adjustment involves updating the specific IP address, which affects how devices are routed through the network. By changing the static IP, the company can ensure proper routing of devices within their /28 network without affecting other networks.
Philco 👍 1 Answered A

.240 = /28 = 16-2 IPs = 16 networks

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