Troubleshooting and Network Diagnostics
A Structured Troubleshooting Method
Start with symptoms
Effective troubleshooting begins by defining the scope of the issue: one host, one subnet, one site, or one application. Then test the simplest plausible causes first.
Layered isolation
Use the OSI model as a checklist. If Layer 1 is broken, higher-layer tests may mislead you.
Useful tools
Common tools include ping, traceroute, ipconfig or ifconfig, nslookup or dig, netstat or ss, and packet capture tools such as Wireshark.
Symptoms and likely layers
| Symptom | Possible layer |
|---|---|
| No link light | Layer 1 |
| Wrong MAC or switching issue | Layer 2 |
| Cannot reach remote subnet | Layer 3 |
| Application timeout | Layer 4 or 7 |
If a host can reach its default gateway but not an external website by name, which service should you check first?
Name resolution failures can stop access by hostname even when basic IP connectivity works.
Correct answer: DNS
Why is packet capture useful in troubleshooting?
Packet captures reveal evidence that logs and high-level symptoms may hide.
Correct answer: It shows actual headers, timing, and errors on the wire or interface
What is traceroute mainly used for?
Traceroute helps identify where connectivity or delay problems appear along the route.
Correct answer: Finding the path and hop-by-hop latency to a destination
What is a good first step when diagnosing an intermittent network issue?
Intermittent issues require pattern recognition and evidence before making changes.
Correct answer: Collect timestamps, affected users, and reproducible conditions