CompTIA Network+ (N10-009)Network TroubleshootingHard
A technician is troubleshooting a client's laptop that can browse internal file shares by IP address but cannot access them by hostname, and also cannot browse the internet. The laptop's IP configuration is otherwise correct. Which tool should the technician use FIRST to confirm the root cause?
- Anmap to scan the local subnet for open ports
- Bnslookup to test whether the DNS server responds to queries
- Ctcpdump to capture all traffic on the interface
- DA cable tester to verify the physical link integrity
Show answer & explanationAnswer & explanation
Correct answer: B. nslookup to test whether the DNS server responds to queries
Since the laptop can reach resources by IP but not by hostname, and internet access also fails, the most efficient first step is using nslookup to test whether the configured DNS server is resolving names correctly. This quickly isolates the issue to DNS without needing a full packet capture or physical layer check.
Why the other options are wrong
- A. nmap scans for open ports, which isn't relevant to a name-resolution symptom.
- C. tcpdump could eventually help but is a slower, more complex first step when a targeted DNS test is available.
- D. A cable tester checks physical connectivity, but connectivity by IP already proves the physical link works.
Isolating DNS Issues
When a host can reach resources by IP address but not by name, the fault is almost always DNS resolution rather than a physical or routing problem.
- Symptom pattern: 'works by IP, fails by name' points to DNS
- nslookup and dig are the fastest tools to test DNS resolution
- Physical connectivity is already proven if IP-based access succeeds
Memory trick: If IP works but names don't, blame DNS, not the wire.