Proxmox - Networking - Host Only Network

To connect VMs directly on the host, without sending the traffic to the external world, a host only network can be used.

A virtual bridge interface will need to be created, but there is no need to bind it to a physical network interface.

vi /etc/network/interfaces

Add the following lines to the file:

auto vmbr1
iface vmbr1 inet static
        bridge_ports none
        bridge_stp off
        bridge_fd 0

NOTE: All VMs connected to this interface will be able to talk to each other.