Telnet

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

Restart inetd
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]

For example:
telnet 192.168.1.105

Ending the session
To close the session
exit