Ubuntu Networking :: Configuration For "virtual" Network Interface

I'm having difficulty setting up my desired network infrastructure. Here's the situation:

I'm currently using an Ubuntu box as homeserver to do everything from sharing internet (via NAT), over DHCP & DNS to filesharing and even virtualisation.

However, since my machine is capable of virtualising Astaro Security Gateway properly, I would like to transfer and split up responsibilities.

I want to have my Astaro fully responsible for the internet and act as the gateway, and along with that disconnect my physical box from the internet and put it behind Astaro (basically making it just a member of the internal network).

The difficulty I'm having with this is that Astaro is running virtually on the physical box, and I can't seem to figure out how to configure the external interface. It needs to be up for Astaro to be able to use it, but it should not give the physical machine access to the external network.

I'm thinking I could set it to manual and not give it an ip address, but I'm not sure on this.