I recentrly installed Slackware Linux 10 into a box with static ip address (my first one, its hard here in greece to have dsl internet).

I have a problem with iptables:
I dont know how to tell iptables to block a specific port or port range to the world. Let me explain. This box runs ftp, mail and irc server. But I want these services to run only in the local network and any requests from the .."outside" to be blocked.
how can i achieve this?