Telnet is an unsafe application - it sends all your data (like passwords, keystrokes, -everything- ) unencrypted over your network, which makes it very easy for someone to log everything you do. In other words: 'su' one time, and loose your box.

Instead of telnet, use SSH (protocol 2 is safer than protocol 1). If you are lucky, you might find that the administrators of your network disabled telnet because of its unsafe nature, but did not disable SSH, to provide an safe alternative to telnet (SSH uses another port than telnet).

In any way, if you are connecting to a machine you can configure, it's usually quite easy to configure the SSH daemon to listen on a non-default port. Check the man-pages of your distribution for sshd.

Doing a quick google for 'tunnel ssh' resulted in quite a few hits, so if you're unable or unprivileged to reconfigure the box, you'll probably find something there. Good luck!

On a side-note: don't pay much attention to nickpickers here that either flame you or assign negative antipoints for questions like these. Your question is valid and well constructed, even if it expresses little knowledge of computer-security. Apparently some users here expect all others to be computer geniusses. They tend to forget that everyone was a beginner once, and that there is no such thing as a stupid question.