4.3.1. Troubleshooting Wired and Wireless Network Issues

💡 First Principle: Use command-line tools (ipconfig, ping, tracert) to quickly test and verify different layers of network connectivity, from local IP configuration to routing paths.

Scenario: A user reports "no internet." You sit at their computer to begin troubleshooting.

Technician's Diagnostic Path:

1. Check Local IP Configuration:
   • Action: Open a command prompt and type ipconfig (Windows) or ifconfig (Linux/macOS).
   • Analysis: Do they have an IP address? Is it an APIPA address (169.254.x.x)? If so, they can't reach the DHCP server. An APIPA address is self-assigned from 169.254.0.0/16 (RFC 3927) and comes with no default gateway or DNS server, so the PC can still talk to other devices on the same local subnet/link (such as other 169.254.x.x hosts) but cannot reach the internet, cloud services or servers on other networks until DHCP works again. Is the Default Gateway correct?
2. Test Local Network Connectivity:
   • Action: ping <default_gateway_IP>. For example, ping 192.168.1.1.
   • Analysis: If this works, their local connection to the router is good. The problem is likely beyond the router.
3. Test Internet Connectivity:
   • Action: ping 8.8.8.8 (a public Google DNS server).
   • Analysis: If this works but browsing doesn't, the problem is almost certainly DNS. If this fails, the problem is with the router's connection to the internet or the ISP.
4. Test DNS Resolution:
   • Action: nslookup www.comptia.org.
   • Analysis: If this fails but ping 8.8.8.8 worked, the configured DNS server is down or incorrect.

Reading Ping Results:

• Request timed out: Packets were sent but no reply came back (host down, ICMP blocked, or packet loss). With ping -t (continuous), a mix of replies and timeouts means intermittent connectivity, not a dead host.
• Destination host unreachable: Usually a routing problem: the sender or a router on the path has no route to the destination network.

Lights and Packet Capture:

• Link and activity lights: A link light means the NIC and switch negotiated a physical connection; the activity light blinks as data moves. Link on with no activity: connected, but nothing is being transmitted. No link light: unplugged or bad cable, dead port, or a failed NIC.
• Protocol analyzer (packet sniffer, e.g. Wireshark): Captures and decodes the actual packets on the wire; a network tap only copies the traffic for such a tool to read.

Common Symptom-to-Cause Patterns:

• Ping to an IP works, but the name fails: DNS. If DNS works for other computers, clear the local cache with ipconfig /flushdns.
• Ping to the web server's IP works but the site will not load in any browser: A firewall is blocking the web port (80/443). A successful ping proves only that ICMP gets through.
• Ping works by IP and by name, but one user cannot open a share that others can: The network is fine; check that user's permissions and credentials on the share.
• Browser cannot load sites but other internet apps work: Check the browser's proxy settings.
• Duplicate IP address: Intermittent connectivity and an IP address conflict warning on one or both devices. Find the statically configured duplicate, or release and renew the DHCP lease. The fe80:: IPv6 link-local address is always present on an IPv6-enabled adapter and is not a sign of DHCP failure.
• Slow with many collisions despite a gigabit switch and good cabling: Duplex mismatch between the NIC and the switch port.
• Only one floor or area is slow: Check the switch or AP serving that area first. The ISP link, firewall rules, or DNS would affect everyone.
• Drops for a few seconds when the desk or chair moves: Poor cable terminations or connectors being disturbed; inspect and re-terminate.
• Suspected bad switch port after trying a different patch cable: Move the connection to a known-good port on the same switch. Change one variable at a time; replacing the switch, updating firmware, or rebooting the PC are not the next step.

Ordered Procedures:

• Laptop cannot join the Wi-Fi: (1) Confirm the wireless adapter is enabled (Airplane mode off, hardware switch, Device Manager); (2) confirm the correct SSID is visible with enough signal; (3) re-enter the passphrase or forget and re-add the network profile; (4) check IP assignment with ipconfig and ping the default gateway, escalating if those fail.
• Wired connection drops several times an hour: (1) Inspect and reseat the patch cable; (2) swap in a known-good cable and see whether drops persist; (3) test the wall jack with a cable tester or connect directly to the switch to bypass the wall run; (4) check switch port logs/interface counters and try a different port.

⚠️ Common Pitfall: Assuming a "no internet" problem is an ISP outage. It is far more likely to be a local issue with the user's computer, a cable, the local switch, or the SOHO router.

Key Trade-Offs:

• Wired vs. Wireless Troubleshooting: Wireless adds layers of complexity like signal strength, interference, and encryption that are not present in wired troubleshooting.

Reflection Question: You are troubleshooting a "no internet" issue. You use ipconfig and see the user has an IP address of 169.254.10.20. What does this tell you, and what is your next troubleshooting step?