Network+ N10-009 Domain 5: Network Troubleshooting — 12 practice questions with answers

Domain 5 is 24% of the CompTIA Network+ (N10-009) exam, which works out to roughly 22of the up to 90 questions on the real test. Below are study notes for the domain's objectives, then every question from our free Network+ N10-009 practice test that targets this domain, with the correct answer and the reasoning behind it. Try to answer each one before reading the explanation.

Objectives in this domain

5.1 Explain the troubleshooting methodology
5.2 Given a scenario, troubleshoot common cabling and physical interface issues
5.3 Given a scenario, troubleshoot common issues with network services
5.4 Given a scenario, troubleshoot common performance issues
5.5 Given a scenario, use the appropriate tool or protocol to solve networking issues

What to focus on

Troubleshooting is the heaviest domain at 24%, and almost every question is a scenario. Memorise the methodology in order: identify the problem, establish a theory of probable cause, test the theory, establish a plan of action, implement the solution or escalate, verify full system functionality and implement preventive measures, then document findings. Many questions simply ask for the next step.

Physical issues (5.2) show up as symptoms: CRC errors and runts point to a duplex mismatch or damaged cable, a link that only negotiates 100 Mbps suggests missing pairs, and a split pair passes continuity but fails under load. Service issues (5.3) are diagnosed by elimination. If a site works by IP but not by name, it is DNS; if a host has a 169.254 address, DHCP failed; if the gateway answers but remote subnets do not, check the subnet mask or routes.

Performance (5.4) separates latency, jitter, packet loss and congestion, and covers wireless signal levels: around -67 dBm or better is the usual target for voice and data. Tools (5.5) pair with jobs: ping and traceroute for reachability and path, nslookup or dig for DNS, netstat or ss for listening ports, a tone generator for finding a cable, a cable tester for wiring faults and an OTDR for breaks in fibre.

Practice questions and explanations

1. A technician has identified that a failed switch uplink caused an outage and has replaced the cable. According to the CompTIA troubleshooting methodology, what should be done NEXT?
1. A) Document findings, actions and outcomes
2. B) Establish a theory of probable cause
3. C) Verify full system functionality and implement preventive measures
4. D) Question users to identify symptoms
Answer: C — Verify full system functionality and implement preventive measures
The methodology runs: identify the problem, establish a theory, test the theory, establish a plan of action, implement the solution, verify full functionality and implement preventive measures, then document. Having implemented the fix, verification comes next; documentation is the final step. (Objective 5.1)
2. A newly installed Cat 6 run shows a link but intermittent errors and reduced speed. A cable tester reports that pins 3 and 6 are split across different pairs. What is the fault?
1. A) Open pair
2. B) Short
3. C) Split pair
4. D) Reversed pair
Answer: C — Split pair
A split pair means the two wires of a signalling pair were terminated on wires from different twisted pairs; continuity exists so the link comes up, but the loss of the pair's twist causes crosstalk and errors. An open shows no continuity, a short joins two conductors, and a reversed pair swaps the two wires of one pair at one end. (Objective 5.2)
3. A gigabit-capable PC connects to a gigabit switch but only negotiates 100 Mbps. The patch cable is old. What is the MOST likely cause?
1. A) The switch port is administratively down
2. B) The cable only has two pairs connected (or is Cat 5), which limits it to 100 Mbps
3. C) The PC has the wrong default gateway
4. D) A duplex mismatch
Answer: B — The cable only has two pairs connected (or is Cat 5), which limits it to 100 Mbps
1000BASE-T needs all four pairs; a cable with only pairs 1-2 and 3-6 terminated, or damaged pairs, can still link at 100BASE-TX, which uses two pairs. An administratively down port would show no link at all, a gateway error affects routing, not link speed, and a duplex mismatch causes collisions and errors rather than a clean 100 Mbps negotiation. (Objective 5.2)
4. A user can reach websites by IP address but not by name. Which service is MOST likely failing?
1. A) DHCP
2. B) DNS
3. C) NTP
4. D) ARP
Answer: B — DNS
Reaching a site by IP proves connectivity and routing work; failing by name isolates the problem to name resolution, which is DNS. nslookup or dig would confirm it. DHCP gave the client an address (it has connectivity), NTP affects time, and ARP resolves IPs to MACs on the local link. (Objective 5.3)
5. Several hosts on one subnet have the same IP address as hosts on another subnet, and they can no longer reach each other. A technician finds that a second, unauthorised DHCP server is handing out addresses. What is this problem called?
1. A) DHCP scope exhaustion
2. B) Rogue DHCP server
3. C) Incorrect default gateway
4. D) Duplicate MAC address
Answer: B — Rogue DHCP server
A rogue DHCP server (often a misconfigured home router plugged into the network) offers addresses from the wrong scope, causing duplicates, wrong gateways and broken connectivity. Scope exhaustion means the legitimate server has run out of leases, while gateway and MAC issues are different symptoms. DHCP snooping on the switches prevents it. (Objective 5.3)
6. A host can ping its default gateway but cannot reach any device on another subnet. The gateway can reach those subnets. Which host setting is MOST likely wrong?
1. A) The subnet mask
2. B) The DNS server
3. C) The MAC address
4. D) The NTP server
Answer: A — The subnet mask
If the host's mask is too large (for example /16 instead of /24), it believes remote subnets are local and ARPs for them instead of sending traffic to the gateway, so the gateway is reachable but remote hosts are not. DNS would break names, not IP reachability; MAC and NTP settings do not affect routing decisions. (Objective 5.3)
7. Voice calls over the WAN are choppy. Monitoring shows packets arriving with widely varying delays. Which metric describes this condition?
1. A) Latency
2. B) Jitter
3. C) Packet loss
4. D) Bandwidth
Answer: B — Jitter
Jitter is variation in packet arrival time, which disrupts real-time audio because the receiver's buffer runs dry or overflows. Latency is the average delay itself, packet loss is packets that never arrive, and bandwidth is capacity. QoS prioritisation for voice is the usual fix. (Objective 5.4)
8. A switch interface shows a rapidly increasing count of CRC errors and runts. Which issue is MOST likely?
1. A) A duplex mismatch or a damaged cable
2. B) An incorrect VLAN assignment
3. C) A DNS misconfiguration
4. D) An expired DHCP lease
Answer: A — A duplex mismatch or a damaged cable
CRC errors and runts (frames shorter than 64 bytes) are physical or data-link symptoms: collisions from a duplex mismatch, or corruption from a damaged cable, bad connector or interference. VLAN, DNS and DHCP problems stop traffic flowing correctly but do not corrupt frames on the wire. (Objective 5.4)
9. Wireless clients at the far end of an office lose connectivity while clients near the access point are fine. A site survey shows the far area at -85 dBm. What is the problem?
1. A) Channel overlap
2. B) Insufficient signal strength (coverage hole)
3. C) Wrong passphrase
4. D) Captive portal timeout
Answer: B — Insufficient signal strength (coverage hole)
Received signal strength of -85 dBm is too weak for reliable data; most designs target around -67 dBm or better for voice and data. The fix is another access point or repositioning, not a channel change. Channel overlap degrades throughput everywhere, and a wrong passphrase or portal issue would affect clients regardless of location. (Objective 5.4)
10. A technician needs to see every router hop between a workstation and a remote server to find where latency is introduced. Which tool should be used?
1. A) ping
2. B) traceroute / tracert
3. C) ipconfig / ifconfig
4. D) nslookup
Answer: B — traceroute / tracert
traceroute (tracert on Windows) lists each hop and its round-trip time by sending packets with increasing TTL values, so the hop where delay jumps becomes visible. ping only measures end-to-end reachability and latency, ipconfig shows local addressing, and nslookup queries DNS. (Objective 5.5)
11. A technician suspects a server's application is not listening on the expected port. Which command shows the TCP ports a host is currently listening on?
1. A) netstat (or ss)
2. B) arp -a
3. C) tracert
4. D) dig
Answer: A — netstat (or ss)
netstat (and its modern Linux replacement ss) lists listening sockets and active connections with their ports and states. arp -a shows the IP-to-MAC cache, tracert maps the path to a destination, and dig performs DNS queries. (Objective 5.5)
12. A technician must locate which of several unlabeled cables in a patch panel runs to a particular wall jack, without disconnecting users. Which tool is designed for this?
1. A) Loopback plug
2. B) Tone generator and probe
3. C) Optical time-domain reflectometer
4. D) Spectrum analyser
Answer: B — Tone generator and probe
A tone generator puts an audible signal on the cable at the jack and the probe picks it up at the patch panel, identifying the run. A loopback plug tests a port, an OTDR locates faults in fibre, and a spectrum analyser visualises RF interference. (Objective 5.5)

Reading answers is not the same as recalling them. When you have worked through every domain, take the timed Network+ N10-009 practice test: it shuffles the questions and options and scores you by domain, so you can see whether this one still needs work.

Network+ N10-009 Domain 5: Network Troubleshooting — 12 practice questions with answers

Objectives in this domain

What to focus on

Practice questions and explanations

1. A technician has identified that a failed switch uplink caused an outage and has replaced the cable. According to the CompTIA troubleshooting methodology, what should be done NEXT?

2. A newly installed Cat 6 run shows a link but intermittent errors and reduced speed. A cable tester reports that pins 3 and 6 are split across different pairs. What is the fault?

3. A gigabit-capable PC connects to a gigabit switch but only negotiates 100 Mbps. The patch cable is old. What is the MOST likely cause?

4. A user can reach websites by IP address but not by name. Which service is MOST likely failing?

5. Several hosts on one subnet have the same IP address as hosts on another subnet, and they can no longer reach each other. A technician finds that a second, unauthorised DHCP server is handing out addresses. What is this problem called?

6. A host can ping its default gateway but cannot reach any device on another subnet. The gateway can reach those subnets. Which host setting is MOST likely wrong?

7. Voice calls over the WAN are choppy. Monitoring shows packets arriving with widely varying delays. Which metric describes this condition?

8. A switch interface shows a rapidly increasing count of CRC errors and runts. Which issue is MOST likely?

9. Wireless clients at the far end of an office lose connectivity while clients near the access point are fine. A site survey shows the far area at -85 dBm. What is the problem?

10. A technician needs to see every router hop between a workstation and a remote server to find where latency is introduced. Which tool should be used?

11. A technician suspects a server's application is not listening on the expected port. Which command shows the TCP ports a host is currently listening on?

12. A technician must locate which of several unlabeled cables in a patch panel runs to a particular wall jack, without disconnecting users. Which tool is designed for this?