- A
Incorrect DNS server configuration on the workstation
Why wrong: DNS is used for name resolution, not for IP-level connectivity. Pinging an IP address does not use DNS, so this would not explain a failure to ping 8.8.8.8.
- B
Missing default route on the router
A missing default route on the router prevents packets destined for external networks from being forwarded beyond the router, even though the workstation can reach the router itself.
- C
Incorrect subnet mask on the workstation
Why wrong: An incorrect subnet mask would affect the workstation's ability to determine whether a destination is local or remote, but the user can already ping internal hosts, making this unlikely.
- D
Malware on the workstation is blocking ICMP traffic
Why wrong: If malware were blocking ICMP, the ping to the gateway would also likely fail or be inconsistent. The ability to ping internal servers suggests ICMP is not blocked locally.
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 technician is troubleshooting a user's inability to access the internet. The user can successfully ping the default gateway and internal servers, but cannot ping a public IP address such as 8.8.8.8. The technician checks the firewall logs and confirms that outbound ICMP traffic to 8.8.8.8 is permitted. Which of the following is the most likely cause of the 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.
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
Missing default route on the router
The user can ping the default gateway and internal servers, which confirms that Layer 2 and Layer 3 connectivity within the local network is working. However, the inability to ping a public IP (8.8.8.8) indicates that traffic is not being forwarded beyond the local subnet. A missing default route on the router means the router does not know where to send packets destined for external networks, so it drops them. Since outbound ICMP is permitted on the firewall, the issue is routing, not filtering.
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.
- ✗
Incorrect DNS server configuration on the workstation
Why it's wrong here
DNS is used for name resolution, not for IP-level connectivity. Pinging an IP address does not use DNS, so this would not explain a failure to ping 8.8.8.8.
- ✓
Missing default route on the router
Why this is correct
A missing default route on the router prevents packets destined for external networks from being forwarded beyond the router, even though the workstation can reach the router itself.
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.
- ✗
Incorrect subnet mask on the workstation
Why it's wrong here
An incorrect subnet mask would affect the workstation's ability to determine whether a destination is local or remote, but the user can already ping internal hosts, making this unlikely.
- ✗
Malware on the workstation is blocking ICMP traffic
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a firewall rule is blocking traffic when the symptom is a ping failure, but the question explicitly states ICMP is permitted, shifting the focus to routing; Cisco tests the distinction between policy-based blocking and routing-based unreachability.
Detailed technical explanation
How to think about this question
A default route (0.0.0.0/0) is the entry in the routing table that tells the router where to send packets when no more specific route matches the destination. Without it, the router will generate an ICMP Destination Unreachable (Network Unreachable) message for any external IP. In enterprise networks, the default route often points to a next-hop ISP router or a border firewall, and its absence is a common misconfiguration after a router reboot or static route deletion.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
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: Missing default route on the router — The user can ping the default gateway and internal servers, which confirms that Layer 2 and Layer 3 connectivity within the local network is working. However, the inability to ping a public IP (8.8.8.8) indicates that traffic is not being forwarded beyond the local subnet. A missing default route on the router means the router does not know where to send packets destined for external networks, so it drops them. Since outbound ICMP is permitted on the firewall, the issue is routing, not filtering.
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 11, 2026
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