- A
Server A's firewall is blocking incoming ICMP
A firewall on Server A can permit outgoing replies while blocking incoming requests, causing the one-way ping failure.
- B
Server B's firewall is blocking outgoing ICMP
Why wrong: If Server B's firewall blocked outgoing ICMP, it would not send the ping request, and Server A would not receive the ping attempt. The symptom would be 'destination unreachable' from Server B, but Server A would see no request. However, Server A can ping Server B, so Server B replies are allowed in. Blocking only outgoing requests on Server B would result in a different failure (Server B cannot initiate pings, but could still receive them). Given the symptom, blocking incoming on Server A is more common.
- C
The cable connecting Server A is faulty
Why wrong: A faulty cable would likely cause intermittent or complete connectivity loss in both directions, not a one-way failure.
- D
There is a duplex mismatch on Server B's switch port
Why wrong: Duplex mismatch typically causes high error rates, collisions, and poor performance in both directions, not a complete failure of one-way communication.
Quick Answer
The answer is Server A's firewall blocking incoming ICMP. This is correct because the one-way ping troubleshooting scenario reveals a classic host firewall issue: Server A can initiate pings to Server B, meaning its outbound ICMP traffic is permitted and Server B’s replies are allowed back, but Server B’s pings to Server A fail because Server A’s firewall is configured to deny inbound ICMP echo requests. On the CompTIA Network+ N10-009 exam, this question tests your ability to distinguish between network-layer problems and host-based firewall rules, especially when both servers share the same VLAN and subnet with no intermediary ACLs. A common trap is assuming a switch or routing issue, but the identical switch ports and direct connectivity point squarely to the endpoint’s own security settings. Remember the memory tip: “Outbound works, inbound blocked—check the host’s firewall first.”
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 Server A can ping Server B by IP address, but Server B cannot ping Server A. Both servers are in the same VLAN and subnet, connected to the same switch. The switch ports are configured identically, and there are no ACLs or firewalls between them. 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.
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
Server A's firewall is blocking incoming ICMP
Server A can ping Server B by IP address, meaning ICMP echo requests from Server A reach Server B and echo replies return successfully. However, Server B cannot ping Server A, which indicates that ICMP echo requests from Server B are not reaching Server A or their replies are blocked. Since both servers are in the same VLAN/subnet with no ACLs or firewalls between them, the most likely cause is that Server A's host-based firewall is blocking incoming ICMP (echo requests), preventing Server B's pings from being processed. This is a classic symptom of a one-way firewall rule that permits outbound ICMP but denies inbound ICMP.
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.
- ✓
Server A's firewall is blocking incoming ICMP
Why this is correct
A firewall on Server A can permit outgoing replies while blocking incoming requests, causing the one-way ping failure.
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.
- ✗
Server B's firewall is blocking outgoing ICMP
Why it's wrong here
If Server B's firewall blocked outgoing ICMP, it would not send the ping request, and Server A would not receive the ping attempt. The symptom would be 'destination unreachable' from Server B, but Server A would see no request. However, Server A can ping Server B, so Server B replies are allowed in. Blocking only outgoing requests on Server B would result in a different failure (Server B cannot initiate pings, but could still receive them). Given the symptom, blocking incoming on Server A is more common.
- ✗
The cable connecting Server A is faulty
Why it's wrong here
A faulty cable would likely cause intermittent or complete connectivity loss in both directions, not a one-way failure.
- ✗
There is a duplex mismatch on Server B's switch port
Why it's wrong here
Duplex mismatch typically causes high error rates, collisions, and poor performance in both directions, not a complete failure of one-way communication.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the misconception that a firewall blocking outgoing ICMP on Server B would cause the symptom, but the correct reasoning is that the blocking must be on the target server (Server A) for incoming ICMP, creating a one-way ping scenario.
Detailed technical explanation
How to think about this question
Windows Firewall, by default, blocks inbound ICMPv4 echo requests (ping) while allowing outbound ICMP, which creates this exact asymmetric behavior. On Linux, iptables or nftables rules can similarly permit outbound ICMP but drop inbound echo requests. The ICMP protocol (RFC 792) does not differentiate between request and reply at the network layer, so a host-based firewall must inspect the ICMP type field (type 8 for echo request, type 0 for echo reply) to apply selective filtering.
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: Server A's firewall is blocking incoming ICMP — Server A can ping Server B by IP address, meaning ICMP echo requests from Server A reach Server B and echo replies return successfully. However, Server B cannot ping Server A, which indicates that ICMP echo requests from Server B are not reaching Server A or their replies are blocked. Since both servers are in the same VLAN/subnet with no ACLs or firewalls between them, the most likely cause is that Server A's host-based firewall is blocking incoming ICMP (echo requests), preventing Server B's pings from being processed. This is a classic symptom of a one-way firewall rule that permits outbound ICMP but denies inbound ICMP.
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
This N10-009 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the N10-009 exam.
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