Articles
Short explanations linked to the tools: how to calculate, how to troubleshoot and what to avoid.
SSL certificate errors: what they mean and how to fix them
An expired certificate, a missing intermediate, a name mismatch and old TLS, the most common browser warnings and their causes on the server side.
Why will a website not load? Ping and traceroute step by step
How to read ping and traceroute output, what asterisks and latency spikes mean, and how to tell whether the problem is on your side, at your provider or at the server.
How to check whether a port is open (e.g. after port forwarding on a router)
Open, closed and filtered ports, router port forwarding, firewalls and CGNAT, the most common reasons why "nothing works".
Why does my e-mail go to spam? DNSBL, PTR, SPF, DKIM and DMARC
How the receiving server judges the sender, and what to check step by step before you start blaming the message content.
Basic DNS records: A, AAAA, CNAME, MX and TXT
What each of the five most common records is for, what TTL means and why a DNS change "does not work straight away".