general OSX IPSec question

I am trying to get my companies VPN solution to work on my mac. Currently we have a Windows based client which connects to a VPN gateway using standard IPSec. I've gotten most everything worked out but there is one thing left that I'm not sure how to do. The VPN assigns each client a local IP address for the private network. Suppose the network is 100.0.0.0/8 you might be given 100.0.0.10 as your IP address. The encapsulated packets need to use this IP address as the source address of my machine. I'm not really sure how to do this. Do I need to use a gif interface? Tim