multihoming/IP aliasing


how can I set up my OSX to work on multhoming or IP aliasing, as It was possible on 0S9 ?
I have a 4Dserver and a 4D client on the same UC, and I need to give two differents IPs to work with.
Under OS9, I had to write a "Secondary IP Adresses" textfile in my Preferences folder, but now ?
thanks for all. It works, of course, but not with the 4D server, due to problems of the software itself.
I spend some days to finally know it, as you can see.

thanks again.