[stephen_g]

VMs usually have their virtual NICs connected to a bridge interface on the host (like a virtual switch) so they can communicate. Proxmox creates one up by default that is also bridged to the physical NIC you set up for management when you install it, so it just works.

In the router case, you'd likely want this default one to be the 'internal' network and have a separate interface (either physical or VLAN) for the WAN.