We have 2 separeted networks in the company, I just wanted to redirect an extern querry from the internet, to a webserver located on the second intranet.
On windows I've been using Nat32.exe as a Router. It's quite easy, just set up all the standard rules, select a network card. Then choose config/application mapping, you have to chose a port and an Ipaddress to forward, if the ports are not standard, you'd better double-click on the application name to change the related script. the syntax is as follows.
exec "ppmap add tcp $port $addr $port"
On a linux machine, you can edit iptable. directly, or use a firewall/router program like firestarter.
Go into policy, allow connections from host, forward service, for example: web on firewall port 8000 to IP adress port 80.
It's not realy difficult. I just had a problem because I was using a hostname, it worked only with the IP address.