Iptables help?

[Jake]

I have my linux box set up with iptables for NAT but for the life of me I
cannot figure out how to do port forwaring.
Perhaps its simple and I overlooked it, but all I want is to have ports
forward to the internal ips of the boxes for a given port.

So if you want to ftp into my linux box it really forwards the request to my
ftp server behind the linux box on the private network.

Anyone have any pointers?

-
Jake