Networking Basics - Telnet

Nowadays, Telnet is an utility for manually debugging connectivity issues.
Historically it was a network protocol developed in 1969.
You can use it to manually connect and talk to network services to establish they are functioning e.g SMTP, FTP, SQL Server

Use : TELNET [hostname/ip] [port]

Examples :

TELNET ftpserver 21

TELNET smtpserver 25

TELNET sqlserver 1433