Telnet is a network protocol, used to provide bi-directional text-oriented communication facility.

Note: Telnet in is insecure protocol and it is recommended that you use ssh server. 

WARNING! Installing telnet on your server makes the server open to an un-encrytpted communications, it’s not recommended to use telnet, use Secure (SSH).

Telnet Server installation

In Debian/Ubuntu
user@techinx$ sudoapt-get install telnetd
In fedora/RedHat
root@techinx# yum install telnet-server telnet xinetd

Configure telnet server (turn on telnet server)

If you are using Red Hat / Fedora Linux
The configuration file for telnet is /etc/xinetd.d/telnet. To enable telnet server you need to open this file and make sure disable = no read as disable = yes.
Alternately,
root@techinx# chkconfig telnet on
To start telnet server type command:
root@techinx# /etc/init.d/xinetd restart

How to enable root login from telnet server

Edit /etc/securetty file, In the end of file add pts/0 to enable one telnet session for root. if you need to open more telnet session for root and add more pts/1 pts/2 and so on.

Restart services, you are done!.