Been a while since I worked on virtualbox. You need to go to the VBox network settings and set up the a virtual adapter for host-only and one set to bridged.
So if you connect your pc through the firewall you would be on an internal network and the bridged adapter will act as the external same as a server with 2 NICs.
Hope thats what you were looking for? Just set the host-only on its own IP range the bridged adapter would use whatever IP range your router is on.
Oops, and yeah. Only one NIC.
Lol keep forgetting stuff. Red you'll connect to the bridged adapter and green to host.