Checking connectivity.
WHAT ip DOES
Shows and configures addresses, routes and interfaces.
WHAT REPLACED WHAT
It replaced the older ifconfig and route commands.
WHAT TO CHECK FIRST
Whether the interface has an address Whether there is a default route Whether DNS resolves
WHY IN THAT ORDER
Each depends on the one before.
WHAT ping TESTS
Whether a host responds to a basic probe.
WHAT IT DOES NOT PROVE
That a service on that host is working.
WHY
Many hosts block probes entirely.
WHAT traceroute AND mtr SHOW
The path to a destination, and where delay or loss occurs.
WHY mtr IS BETTER
It runs continuously, showing loss over time rather than one sample.
WHAT dig DOES
Queries DNS directly.
WHY IT BEATS ping FOR DNS QUESTIONS
It shows exactly what was answered, by which server.
WHAT ss DOES
Lists network connections and listening ports.
WHAT TO USE IT FOR
Confirming a service is listening, and on which address.
WHY THE ADDRESS MATTERS
A service listening only on the loopback address is unreachable from outside.
WHAT curl DOES
Makes requests, showing headers and response.
WHY IT IS THE BEST WEB DIAGNOSTIC
It tests the server directly, without a browser's caching and interference.
