Question 9 of 520

Network Troubleshooting →mediumMultiple ChoiceObjective-mapped

N10-009 Network Troubleshooting Practice Question

This N10-009 practice question tests your understanding of network troubleshooting. 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 network technician is troubleshooting an issue where users on VLAN 20 can access internal servers and the default gateway but cannot reach any external websites. The router's ACL is configured to permit all outbound traffic. What is the most likely cause of this issue?

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

Open the full VLAN trunking answer →

A
The DNS server is unreachable from the VLAN
Why wrong: DNS issues would affect name resolution, but the users can access internal resources by IP. Additionally, the problem is specifically with external websites, and if DNS were the root cause, they would likely be unable to resolve any hostnames, not just external ones.
B
The DHCP server is not providing a default gateway
Why wrong: A missing default gateway would prevent access to any network beyond the local subnet, including internal servers. Since internal servers are accessible, the default gateway is correctly configured.
C
NAT is not configured or is misconfigured on the router
NAT is required to translate private VLAN IPs to a public IP for internet access. Even if ACLs permit traffic, without proper NAT, packets will not be forwarded to the internet. This matches the symptom of being able to reach internal resources but not external ones.
D
The switch port for the router is in the wrong VLAN
Why wrong: If the switch port were in the wrong VLAN, the router would not be reachable, but users can ping the default gateway. This indicates Layer 2 connectivity is correct.

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

✓

NAT is not configured or is misconfigured on the router

Since users can access internal servers and the default gateway, Layer 3 connectivity within the VLAN is working, and the router's ACL permits all outbound traffic. The inability to reach external websites while internal resources are reachable strongly indicates that Network Address Translation (NAT) is either not configured or misconfigured on the router. Without NAT, private IP addresses from VLAN 20 cannot be translated to a public IP, so return traffic from external web servers has no route back to the internal hosts.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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 is unreachable from the VLAN
Why it's wrong here
DNS issues would affect name resolution, but the users can access internal resources by IP. Additionally, the problem is specifically with external websites, and if DNS were the root cause, they would likely be unable to resolve any hostnames, not just external ones.
✗
The DHCP server is not providing a default gateway
Why it's wrong here
A missing default gateway would prevent access to any network beyond the local subnet, including internal servers. Since internal servers are accessible, the default gateway is correctly configured.
✓
NAT is not configured or is misconfigured on the router
Why this is correct
NAT is required to translate private VLAN IPs to a public IP for internet access. Even if ACLs permit traffic, without proper NAT, packets will not be forwarded to the internet. This matches the symptom of being able to reach internal resources but not external ones.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The switch port for the router is in the wrong VLAN
Why it's wrong here
If the switch port were in the wrong VLAN, the router would not be reachable, but users can ping the default gateway. This indicates Layer 2 connectivity is correct.

Common exam traps

Common exam trap: answer the scenario, not the keyword

CompTIA often tests the misconception that an ACL permitting all outbound traffic is sufficient for internet access, but without NAT, private IP addresses are not routable on the public internet, so the ACL alone cannot fix the issue.

Detailed technical explanation

How to think about this question

NAT (specifically PAT or overload) is required to translate private RFC 1918 addresses (e.g., 10.x.x.x, 172.16-31.x.x, 192.168.x.x) to a public IP for internet access. A common misconfiguration is forgetting to include the 'ip nat inside' and 'ip nat outside' interface commands, or not applying the ACL to the NAT translation rule, causing traffic to be dropped or not translated. In real-world scenarios, a missing 'ip nat inside source list 1 interface GigabitEthernet0/0 overload' command is a frequent oversight that leads to this exact symptom.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this N10-009 question test?

Network Troubleshooting — This question tests Network Troubleshooting — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: NAT is not configured or is misconfigured on the router — Since users can access internal servers and the default gateway, Layer 3 connectivity within the VLAN is working, and the router's ACL permits all outbound traffic. The inability to reach external websites while internal resources are reachable strongly indicates that Network Address Translation (NAT) is either not configured or misconfigured on the router. Without NAT, private IP addresses from VLAN 20 cannot be translated to a public IP, so return traffic from external web servers has no route back to the internal hosts.

What should I do if I get this N10-009 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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?

Read the scenario before looking for a memorised answer.

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Last reviewed: Jun 30, 2026

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