How to install telnet server in Ubuntu/debian ?

SSH (ssh and sshd) is much, much, much more secure, as telnet sends the password in clear text over the network. No, that is not secure when you have more than one computer on the net. You could as well post your login and password in your window, in large letters.
If you need telnetd, you have more then one computer on your net.

IF you need telnet, just install the client, not the server. And remember that all you write, is in plain text for everyone on all nets between your computer and the target. So do not use telnet for logging in somewhere.

(and the preferd way to restart a service is ‘sudo invoke-rc.d telnetd’)

i am still not able to telnet the machinde, my system is ubuntu 12.04, and i have installed telnetd