The summary.
THE FOUR QUESTIONS
What exactly fails? When did it start? What changed? Who is affected?
Most faults follow a change, and documented changes let you answer instantly.
THE SEQUENCE THAT LOCATES ALMOST ANYTHING
Link, address, gateway, external address by number, name resolution, then the service.
Each depends on the one before it. Work upward and stop where it fails.
THE SINGLE MOST USEFUL SEPARATION
Test an external address by number, not by name.
That separates connectivity from DNS in one step.
THE SECOND MOST USEFUL
Test on a cable, and test on a different connection entirely.
Those separate wireless from connection, and your network from the destination.
WHAT TWO ERRORS MEAN
Refused: the host is reachable and the service declined. Timed out: nothing answered.
They point at completely different causes.
BEFORE BLAMING A CERTIFICATE
Check the clock.
FOR INTERMITTENT FAULTS
Record every occurrence with times. Patterns emerge from records and never from memory, and without them intermittent faults are dismissed.
WHEN ESCALATING
Say what you ruled out. That is what turns a report into a resolution.
