hi to all

1)How can we block telent on open ports like 80,23,25 etc..? I need a detailed answer of this question.

thnx
rabeea

You can't.
And why would you want to?
If you have e.g port 80 open for http connection, one can telnet to your port 80. But only via the http protocol, since you only have a socket for http connections listening on port 80.
The only security risk is to have a telnet server daemon running on your system, since telnet uses not an encrypted connection and gives full control to your system.
But as long as you havent a telnet server running, noone can access your computer via the telnet protocol.

So if you want to allow certain protocols to access your pc, everyone can telnet those protocols (thats the way it has to be, otherwise no other client could connect to your pc, as telnet on a specific port gives you just the possibility to use the protocol specified on that port).

Telnet is just a "low tech" client that is being used to connect to the port and provide an interactive session. Telnet itself is not any more dangerous than the "real" clients typically associated with a particular service. Are you experiencing a specific problem?