Here’s a simple howto for setting up Proxmox VE host on 2 ethernet devices looking into local network and the internets. ISP gives me with only one IP to use in a /29 network, so I want to create an anonymous (or ipless) bridge to allow my VM to have this very internet address. Here’s some /etc/network/interfaces:
iface lo inet loopback
# bridge to local network with an address for PVE host
iface vmbr0 inet static
# anonymous bridge to the internets with no IP for the host
iface vmbr1 inet manual
Then create a VM with a network bridged through vmbr1 and assign your external internet address (18.104.22.168 for example) inside the guest OS. Okay, you got a VM looking into the internets with your only external IP. Oh, and if you want your VM to look into the local network too, just add an interface bridged through vmbr0 (add the device with PVE management interface). Now you can setup the guest VM to balance IP traffic coming from outside to your local VMs with something like nginx. Much cooler than NAT, right?