Question 846 of 1,020

IP Addressing →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the default gateway on the computers is not set to 10.0.0.1. When multiple users cannot access the internet but can communicate locally, the core issue is that traffic destined for external networks has no exit point; local communication works because it stays within the same subnet and never needs a gateway. This scenario directly tests your understanding of how a default gateway functions as the router interface that forwards packets outside the local network, and on the CompTIA A+ Core 1 220-1201 exam, this is a classic troubleshooting trap where students mistakenly blame DNS or DHCP when the real problem is a missing or incorrect gateway address. Remember that if devices can ping each other but not reach the internet, always verify the gateway first—a simple memory tip is “Local works, internet fails? Check the gateway trails.”

220-1101 IP Addressing Practice Question

This 220-1201 practice question tests your understanding of ip addressing. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.

A technician is troubleshooting a network where several users cannot access the internet, but they can communicate with each other. The network uses a router with IP 10.0.0.1 and a subnet mask of 255.255.255.0. All computers have IPs in the 10.0.0.0/24 range. What is the most likely cause?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

Question 1mediummultiple choice

Review the full subnetting walkthrough →

A
The DNS server address is set to 10.0.0.1.
Why wrong: While DNS could be an issue, the ability to communicate locally suggests the network is working. The lack of internet access points to a default gateway problem.
B
The subnet mask is incorrectly set to 255.255.0.0 on some computers.
Why wrong: An incorrect subnet mask could cause communication issues, but if they can communicate with each other, the subnet mask is likely correct.
C
The default gateway on the computers is not set to 10.0.0.1.
Without the correct default gateway, computers cannot send traffic outside their local subnet, which would prevent internet access.
D
The router's DHCP server is disabled.
Why wrong: If DHCP were disabled, computers might not have IP addresses at all, but they are communicating locally, so they have valid IPs.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The default gateway on the computers is not set to 10.0.0.1.

If users can communicate locally but not access the internet, the issue is likely with the default gateway. The gateway must be correctly configured on each device and the router must have a route to the internet.

Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
The DNS server address is set to 10.0.0.1.
Why it's wrong here
While DNS could be an issue, the ability to communicate locally suggests the network is working. The lack of internet access points to a default gateway problem.
✗
The subnet mask is incorrectly set to 255.255.0.0 on some computers.
Why it's wrong here
An incorrect subnet mask could cause communication issues, but if they can communicate with each other, the subnet mask is likely correct.
✓
The default gateway on the computers is not set to 10.0.0.1.
Why this is correct
Without the correct default gateway, computers cannot send traffic outside their local subnet, which would prevent internet access.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
✗
The router's DHCP server is disabled.
Why it's wrong here
If DHCP were disabled, computers might not have IP addresses at all, but they are communicating locally, so they have valid IPs.

Common exam traps

Common exam trap: usable hosts are not the same as total addresses

Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.

Detailed technical explanation

How to think about this question

Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.

KKey Concepts to Remember

CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.

TExam Day Tips

Write the block size before choosing the subnet.
Check whether the question asks for hosts, subnets or a specific address range.
Do not confuse /24, /25, /26 and /27 host counts.

Key takeaway

Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

Real-world example

How this comes up in practice

A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.

What to study next

Got this wrong? Here's your next step.

Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 220-1201 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this 220-1201 question test?

IP Addressing — This question tests IP Addressing — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The default gateway on the computers is not set to 10.0.0.1. — If users can communicate locally but not access the internet, the issue is likely with the default gateway. The gateway must be correctly configured on each device and the router must have a route to the internet.

What should I do if I get this 220-1201 question wrong?

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

CIDR notation defines the prefix length.

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