Guides
The tools on this site tell you what something is. These pages explain what it means, and what to do next. Written for the moment when something is broken and you need an answer rather than a tutorial.
When something is broken
Start here when it is already failing and you need to narrow it down.
- How to read traceroute outputMost of what looks alarming in a traceroute is normal. Here is which parts actually indicate a fault and which are artefacts of how the tool works.Updated Sep 21, 2026
- A checking order for when the connection is badWork outward from your own machine instead of guessing. Six checks in order, each one ruling out a layer, so you stop at the first that fails.Updated Sep 21, 2026
Names and DNS
Why a name resolves, why it stopped, and why it is different for you.
Foundations
The ideas the rest of it rests on. Worth ten minutes once.