What to read first
If you are checking your own connection, start with the public IP guide and then read about ASN and IPv4 versus IPv6. If you are troubleshooting a domain or website, start with DNS lookup, then port check, then HTTP headers. If a problem only affects certain devices, read the browser information guide to understand what the browser reports.
The guides are written for practical diagnosis rather than theory. Each page explains what the result means, when it matters, common mistakes, and which related tool to use next.