I'll start this series with the simplest and oldest command.
Remember, it is not safe to expose telnet to the internet.
To install on the server (Debian/Ubuntu)
sudo aptitude install telnetd
To start/stop under Debian
Under Debian, the telnet daemon is started by inetd when the port is accessed. This is controlled by the inetd.conf file. If the telnet line is commented out, telnet will not start, otherwise it will start.
After modifying the inetd.conf file you must restart inetd for the changes to take effect.
Comment or uncomment the telnet line
sudo nano /etc/inetd.conf
sudo /etc/init.d/openbsd-inetd restart
To start/stop under RedHat
chkconfig telnet on
chkconfig telnet off
confirming that the port is open
To check that the port is open by name
netstat -l | grep -i ftp
Or faster and more specific, to check by port number
netstat -ln | grep 23
Connecting from the client
Once the server has started, you can connect from the client
telnet [server address]
Ending the session
To close the session