Practical guides
Each guide starts from a concrete problem and covers how to confirm the cause, how to read the results, and the limits. The related tools do the calculations and parsing.
SPF DNS lookup limit (10) and how to trace a permerror
SPF turned into permerror after adding an include, or fails only for some senders: how RFC 7208 counts DNS-lookup terms, how to count them by hand, and how to reduce them.
Ping works but large transfers hang: troubleshooting MTU, MSS, and PMTUD
When page loads or scp stall while ping and SSH work, suspect a Path MTU Discovery failure: confirm it with DF-set pings, compute the MSS, and know what MSS clamping does not fix.