Network Troubleshooting

A network symptom tells you what a user noticed. It does not tell you which component failed. “The website is down” can describe a local link problem, a missing route, failed name resolution, a blocked transport connection, or an application error.

Network troubleshooting turns that broad symptom into a small, testable question. You observe the failure, define its scope, record a baseline, and compare the failing path with a working path. Your goal is to find the first meaningful difference.

Build the path before you test it

Think of communication as a chain:

application
    ↓
name resolution
    ↓
transport and service port
    ↓
IP route and gateway
    ↓
local interface, link, and neighbor

The chain is a troubleshooting map, not a claim that every fault follows a neat layer boundary. A firewall can block a transport port. A bad route can affect only one destination prefix. A working DNS answer can still point to the wrong address.