PC BSD Warden network interface

The documentation for PC BSD's Warden mentions that the configuration of the right network interface is crucial. However, it doesn't say what the "right" interface is. What interface can I use there? The existing physical NIC one? Should I create a virtual one and use that? The aim is to have network access in the jails, i.e. be able to access the internet as well as access services running within the jail from the internet.