Networking two PC's for P2P use need help.

The first is just my main pc and the second pc is going to be my 24/7
Download box. I just have some idea of how to do it, but would like
much more help.

If you could run me though it like for a dummy that would be nice

I have a billion 5102 Four port Ethernet modem. At the moment i have a
network setup between the two pcs and the second pc is shareing my net
conection which is ADSL2-2mbt conection. God only knows how i manged
that

Ive been reading but what stumps me is Port Forwarding how would this
work. I was told to put an IP in for the host machine other than my
Own Internal ip. SO what happens then what do i forward my ports to ?


Totaly confused.

Please be easy on me and explain everything and ASUME nothing

My download box is all ready just have to get this all setup to get it
humming.

Thanks for all your help.