You mentioned however that you are using an Apple Mac OSX machine which means I won't be able to suggest any software for it as I don't know of any that will do the job.
For the Windows platform however there are a number of applications that you can try on your XP machine just to help you get the idea of what they do. A few of them are 'Advanced TCP Mapper', 'Port Reroute' and "Port Mapper."
When you run such applications, you always are required to select an interface/IP Address (in the case you have more than one)and port number that they will 'listen' on and then the IP and port to which they will forward the request.
Take the following scenario as an example:
The main PC that is also the gateway to the Internet has two interfaces, one connected to the Internet (126.96.36.199) and the other to the private LAN (192.168.0.1). We also have an internal webserver running on 192.168.0.2 – a pc on our LAN.
In this scenario, which is similar to yours, we would configure one of the previously mentioned programs to listen on 188.8.131.52 and port 80, and forward all requests arriving to these parameters to 192.168.0.2 and port 80. This way, all users on the Internet have access to the Internal webserver!
Of course there are other ways you can forward requests to the Internal LAN, and one which I use a lot is using a Linux Firewall running IPTables (www.netfilter.org). I won't get into any details as I don't want to confuse you.
I hope the above helps you solve your problem!
This was first published in February 2004