The answer is to check the ACL applied to the router interface. This is correct because the client can reach its default gateway, proving Layer 2 and basic Layer 3 connectivity are functional, and the router’s valid route confirms the routing table knows the path to the remote server’s subnet. Since traffic fails only when crossing the router to a different subnet, the most likely culprit is an Access Control List filtering packets at the router’s interface, blocking the traffic despite a valid route. On the CompTIA Network+ N10-009 exam, this scenario tests your ability to distinguish between routing table issues and security filtering—a common trap is to assume a missing route when the real problem is an ACL silently dropping packets. Remember the mnemonic “Ping Gateway, Not Remote? ACL’s the Note,” reminding you that if the gateway responds but the remote server does not, check for ACLs before reconfiguring routes.
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 connectivity issue. A client can successfully ping its default gateway but cannot ping a server located on a different subnet. The router's routing table shows a valid route to the server's network. What should the technician check NEXT?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL applied to the router interface
Since the client can ping its default gateway but not a server on a different subnet, Layer 3 routing is working (the router has a valid route). The most likely cause is an ACL on the router interface that is blocking traffic to the server's network. ACLs filter traffic based on source/destination IP, protocol, or port, and can permit or deny packets even when a valid route exists.
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 ACL applied to the router interface
Why this is correct
An ACL might be blocking the traffic even though a valid route exists. ACLs are evaluated after the routing decision.
Related concept
Read the scenario before looking for a memorised answer.
✗
The DNS resolution for the server
Why it's wrong here
DNS is not an issue since the client is pinging by IP address.
✗
The ARP cache on the client
Why it's wrong here
The client can reach the gateway, so ARP is working locally.
✗
The duplex settings on the client NIC
Why it's wrong here
Duplex issues would likely affect all traffic, including the ping to the gateway.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the misconception that a valid route in the routing table guarantees end-to-end connectivity, but ACLs can override routing decisions by filtering traffic at the interface level.
Detailed technical explanation
How to think about this question
ACLs are processed in sequential order until a match is found; an implicit deny all at the end can silently drop traffic. The technician should use commands like 'show access-lists' and 'show ip interface' to verify if an ACL is applied inbound or outbound on the router interface facing the client or the server. In real-world scenarios, a misconfigured ACL is a common cause of asymmetric routing issues where pings work one way but not the other.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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 ACL applied to the router interface — Since the client can ping its default gateway but not a server on a different subnet, Layer 3 routing is working (the router has a valid route). The most likely cause is an ACL on the router interface that is blocking traffic to the server's network. ACLs filter traffic based on source/destination IP, protocol, or port, and can permit or deny packets even when a valid route exists.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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