mediumMultiple Choice
DNS Server Unreachable — Troubleshooting Internet Access
A technician is troubleshooting a user's inability to access the internet. The user can ping the default gateway but cannot ping external websites. What is the most likely issue?
⚠ Common exam trap
Many candidates assume a firewall is blocking traffic (Option A) when they see internet failure, but the ability to ping the gateway proves Layer 3 connectivity is intact, pointing instead to a DNS resolution issue.
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
✓
DNS server is not configured or is unreachable
The user can ping the default gateway, which confirms that the local network configuration (IP address, subnet mask, and gateway) is correct and that Layer 3 connectivity to the router is working. However, pinging external websites fails because the DNS server is not configured or unreachable, preventing the resolution of domain names to IP addresses. This is a classic symptom: internal connectivity works, but name resolution fails.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Firewall is blocking all outbound traffic
Why it's wrong here
A blanket outbound block would also prevent the gateway ping, which succeeds here, so it cannot be the cause. It is tempting because firewall rules commonly block internet access, but the correct answer is a DNS resolution failure, since name resolution fails while IP-level routing works.
- ✗
Default gateway is down
Why it's wrong here
A down default gateway would prevent the successful gateway ping the user achieved, so it is ruled out. It is tempting because gateway faults commonly break internet access, but the correct answer is DNS failure, where local routing works yet external names cannot resolve.
- ✗
IP address is misconfigured
Why it's wrong here
A misconfigured IP address would break communication with the default gateway, yet the ping succeeds, so addressing is functional. It is tempting because IP faults commonly cause connectivity loss, but the correct answer is DNS resolution failure, which leaves IP routing intact while name lookups fail.
- ✓
DNS server is not configured or is unreachable
Why this is correct
Pinging the gateway proves local IP connectivity works, so the fault lies above Layer 3 routing. Name resolution fails when no DNS server is configured or reachable, so hostnames for external sites cannot resolve to IP addresses, even though numeric addresses might still respond.
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Same concept, more angles
1 more way this is tested on FC0-U71
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Refer to the exhibit. A user is unable to access the internet but can ping the default gateway. Based on the exhibit, what is the most likely cause?
hard- A.Firewall blocking traffic
- B.Duplicate IP address
- ✓ C.Incorrect DNS server
- D.Incorrect subnet mask
Why C: The user can ping the default gateway, which confirms that Layer 3 connectivity to the local network is working and that the firewall is not blocking ICMP traffic. However, the inability to access the internet (by name or IP) while local connectivity works points to a failure in name resolution. An incorrect DNS server address prevents the client from resolving domain names to IP addresses, which is the most likely cause given the symptoms.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This FC0-U71 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 FC0-U71 exam.