Unit 9 · Network Troubleshooting

9.9Review: key terms and self-check

Key terms

Make sure you can explain each of these in your own words. The glossary has short definitions.

Self-check questions

Write your answers first, then open the answers to check them.

  1. List the steps of the CompTIA-style troubleshooting method in order.
  2. A user can ping 8.8.8.8 but cannot open websites by name. What is the likely fault and how do you confirm it?
  3. Which tool shows where along a path packets stop?
  4. A Linux web server is not responding. Name five checks you would do.
  5. What does an IP address of 169.254.12.5 on a client indicate?
  6. Why should you change only one thing at a time?
Show answers
    1. Identify the problem (gather information, reproduce, define the scope).
    2. Establish a theory of probable cause.
    3. Test the theory to determine the cause.
    4. Establish a plan of action and identify its impact.
    5. Implement the solution (or escalate).
    6. Verify full system functionality and apply preventive measures.
    7. Document findings, actions and outcomes.

  2. DNS failure; confirm with nslookup www.example.com and by changing the DNS server (for example to 8.8.8.8).

  3. tracert (Windows) or traceroute (Linux).

  4. Any five, for example:

    • ping the server and check its IP settings (ip a).
    • systemctl status apache2 to see whether the web service is running.
    • ss -tulpn to check it is listening on port 80/443.
    • sudo ufw status to check the firewall allows the port.
    • journalctl -u apache2 and /var/log/apache2/error.log for error messages; apache2ctl configtest for syntax.
    • df -h, free -h and top for a full disk, low memory or high CPU.
    • nslookup / dig to check the name resolves to the right address.

  5. The client failed to get a DHCP address and self-assigned an APIPA address.

  6. So you can tell which change fixed the problem or caused a new one and can easily undo it.

Next: when you are confident with this unit, test yourself with the multiple-choice questions for Unit 9 in the Final Revision section.