telnet

Connect to TCP ports

medium risk

interactive breakdown — edit the command and press Explain
$ 
⚠ Risks & cautions
What it does

Used today mainly to test whether a TCP port is open.

Examples
telnet example.com 25Test an SMTP port · explain & risk-check →
Related
nc ssh
Open the full explainer →More in Networking

Reference text is hand-written and may be incomplete — check man telnet for your version. Test destructive commands on a non-production system first.