To my knowledge, Netscape does not support telnet directly (internally,
that is). However, you can configure a telnet client as a helper
application so that when you click on a telnet URL, the telnet client is
launched. The NCSA telnet application is probably the most popular one. I
believe it's available for both Mac and Windows. Search one of the big
file archives or try http://www.shareware.com.
By the way, I think there are also "talk" and IRC clients available for
both platforms as well. Good luck!
--- Steve Schacht shot@frontier.net