N10-009 Network Troubleshooting Practice Question
A user reports intermittent inability to access websites. When the issue occurs, the user can ping external IP addresses (e.g., 8.8.8.8) but cannot ping domain names like google.com. The user's IP configuration shows a DNS server address of 8.8.8.8. What is the most likely cause?
⚠ Common exam trap
Watch out — candidates often assume a reachable server (via ping) means all services are working, but CompTIA Network+ exams test the distinction between ICMP reachability and UDP/TCP service availability, especially for DNS where high load can cause intermittent failures.
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
✓
The DNS server is reachable but not responding due to high load.
The user can ping external IP addresses (e.g., 8.8.8.8) but cannot resolve domain names like google.com, which indicates that IP connectivity is working but DNS resolution is failing. Since the DNS server address is 8.8.8.8 and the user can ping it, the server is reachable; however, intermittent failures suggest the server is overwhelmed and dropping or not responding to queries. This matches the symptom of a reachable but unresponsive DNS server due to high load, where ICMP (ping) succeeds but UDP/53 DNS queries time out.
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 reachable but not responding due to high load.
Why this is correct
Correct. If the DNS server is overloaded, it may drop queries intermittently. The user can still ping the server (ICMP is different from DNS), but DNS queries time out.
- ✗
The default gateway is down.
Why it's wrong here
If the default gateway were down, the user would not be able to ping any external IP addresses. The fact that pinging 8.8.8.8 works rules this out.
When this WOULD be correct
A user cannot access any external resources, including pinging external IP addresses, and the default gateway is unreachable from the user's subnet. This would indicate a default gateway failure.
- ✗
The corporate firewall is blocking UDP port 53.
Why it's wrong here
A firewall block on UDP port 53 would cause DNS failures consistently, not intermittently. The user would never be able to resolve domain names.
When this WOULD be correct
In a scenario where a user cannot access any websites and cannot ping any IP addresses or domain names, and network connectivity is otherwise fine, the corporate firewall blocking UDP port 53 would be the likely cause. For example, if the user's DNS server is internal and the firewall blocks all DNS traffic, or if the firewall is misconfigured to drop DNS packets.
- ✗
The user's machine has a corrupted DNS cache.
Why it's wrong here
A corrupted DNS cache typically causes failures for specific cached entries, but the issue is described as intermittent and affects all domain names. Flushing the cache might resolve it, but the intermittent nature points more to server load.
When this WOULD be correct
This option would be correct if the user reported persistent inability to resolve domain names (e.g., always fails to ping google.com) while other network functions work, and flushing the DNS cache resolves the issue. For example, after a malware infection or misconfigured cache entries.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓The DNS server is reachable but not responding due to high load.Correct answer▾
Why this is correct
Correct. If the DNS server is overloaded, it may drop queries intermittently. The user can still ping the server (ICMP is different from DNS), but DNS queries time out.
✗The default gateway is down.Wrong answer — click to see why▾
Why this is wrong here
If the default gateway were down, the user would not be able to ping external IP addresses like 8.8.8.8, but the user can ping them successfully.
★ When this WOULD be the correct answer
A user cannot access any external resources, including pinging external IP addresses, and the default gateway is unreachable from the user's subnet. This would indicate a default gateway failure.
Why candidates choose this
Candidates may assume that any internet connectivity issue is due to the default gateway, overlooking that successful external pings rule out gateway problems.
✗The corporate firewall is blocking UDP port 53.Wrong answer — click to see why▾
Why this is wrong here
The user can ping external IP addresses (8.8.8.8) but not domain names, indicating DNS resolution failure. A firewall blocking UDP port 53 would prevent all DNS queries, making it impossible to ping domain names, but the user's DNS server is 8.8.8.8 (Google's public DNS), which is external. A corporate firewall typically blocks outbound DNS to external servers, but here the issue is intermittent and the DNS server is reachable (since pings to 8.8.8.8 succeed).
★ When this WOULD be the correct answer
In a scenario where a user cannot access any websites and cannot ping any IP addresses or domain names, and network connectivity is otherwise fine, the corporate firewall blocking UDP port 53 would be the likely cause. For example, if the user's DNS server is internal and the firewall blocks all DNS traffic, or if the firewall is misconfigured to drop DNS packets.
Why candidates choose this
Candidates may assume that DNS issues are often caused by firewall blocks, especially in corporate environments. They might overlook that the user can ping the DNS server's IP, which suggests the firewall is not blocking all traffic to that server.
✗The user's machine has a corrupted DNS cache.Wrong answer — click to see why▾
Why this is wrong here
A corrupted DNS cache would cause consistent resolution failures, not intermittent issues. The user can ping external IPs but not domain names, indicating DNS resolution fails intermittently, which points to a server-side problem rather than a local cache issue.
★ When this WOULD be the correct answer
This option would be correct if the user reported persistent inability to resolve domain names (e.g., always fails to ping google.com) while other network functions work, and flushing the DNS cache resolves the issue. For example, after a malware infection or misconfigured cache entries.
Why candidates choose this
Candidates may think DNS cache corruption is a common cause of resolution problems, and the intermittent nature might be misinterpreted as cache corruption that sometimes works. They overlook that cache corruption typically causes consistent failures until cleared.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
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Related to this question
Learn chapter
Troubleshoot: Network Connectivity
Key term
ICMP
ICMP is a network protocol used by devices to send error messages and operational information about network communication problems.
Key term
UDP
UDP (User Datagram Protocol) is a communication protocol that sends data quickly without first checking if the receiver is ready or if the data arrived correctly.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.