This module can be used to give users unique IPs, we use it on our terminal server, so in the logs where only IPs are saved, we still have know who did what, and we can grant internet access on a per user basis. Whatever you might want to do with it, here it is:
The "module" consists of two parts, the kernel module, and the extension for iptables. To build the kernel module, you'll need the source of the kernel you wish to insmod the module into. To build the iptables extension you must first download iptables, and then copy the file into the extensions directory. Then you build iptables and can use the module.
Most likely you'll have to adjust some paths in the Makefile, but all in all, it shouldn't be too dificult.