Firewall hardware requirements 

Generally, your firewall will have a minimum of (2) network interfaces - one for your untrusted (Internet facing) interface and another for your trusted (server facing) traffic. You can build your virtual machine (firewall) with the resources and OS you wish to use and then simply add another network interface, reboot and then add an IP address to your second network interface and you will be up and running. Depending on how you setup your firewall, you may not normally need an IP on the inside interface on your firewall as it will be bridged with the main interface.