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 user reports that they cannot access the internet. The technician is able to ping the default gateway successfully but unable to ping a public IP address (e.g., 8.8.8.8). Which of the following 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.
Why wrong: DNS would affect name resolution, but pinging by IP address does not require DNS. Thus, DNS is not the cause.
B
The router's default route is missing or misconfigured
A missing default route on the router prevents traffic destined for external networks from being forwarded beyond the local subnet.
C
Duplicate IP address on the network
Why wrong: A duplicate IP would typically cause intermittent connectivity or conflicts, but the ability to ping the gateway consistently suggests no IP conflict.
D
Faulty network interface card
Why wrong: A faulty NIC would likely cause failure to communicate with any device, including the gateway.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The router's default route is missing or misconfigured
Successful ping to the default gateway confirms Layer 2 and Layer 3 connectivity within the local subnet, but failure to ping a public IP like 8.8.8.8 indicates that traffic cannot leave the local network. This is most commonly caused by a missing or misconfigured default route on the router, which prevents the router from forwarding packets destined for external networks to the next-hop ISP gateway.
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
Why it's wrong here
DNS would affect name resolution, but pinging by IP address does not require DNS. Thus, DNS is not the cause.
✓
The router's default route is missing or misconfigured
Why this is correct
A missing default route on the router prevents traffic destined for external networks from being forwarded beyond the local subnet.
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.
✗
Duplicate IP address on the network
Why it's wrong here
A duplicate IP would typically cause intermittent connectivity or conflicts, but the ability to ping the gateway consistently suggests no IP conflict.
✗
Faulty network interface card
Why it's wrong here
A faulty NIC would likely cause failure to communicate with any device, including the gateway.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse DNS resolution with IP connectivity and incorrectly choose 'Incorrect DNS server configuration' when the question explicitly states a public IP address is being pinged, not a hostname.
Detailed technical explanation
How to think about this question
A default route (0.0.0.0/0 in IPv4 or ::/0 in IPv6) is the entry in the routing table that matches any destination not explicitly listed. When this route is missing, the router drops packets for external destinations and may send an ICMP Destination Unreachable (Net Unreachable) message back to the host. In real-world scenarios, this often occurs after a router reboot if the static default route was not saved to the startup configuration, or if a dynamic routing protocol like BGP fails to advertise the default route from the ISP.
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 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
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: The router's default route is missing or misconfigured — Successful ping to the default gateway confirms Layer 2 and Layer 3 connectivity within the local subnet, but failure to ping a public IP like 8.8.8.8 indicates that traffic cannot leave the local network. This is most commonly caused by a missing or misconfigured default route on the router, which prevents the router from forwarding packets destined for external networks to the next-hop ISP gateway.
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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