Proxmox private network between KVM and VZ
Solution 1:
on proxmox one can use dummy interface to make a vlan between VM or container or whatever. it will not get out of the server as there is not physical link for.
just put in your /etc/network/interface something like
auto vmbr2
iface vmbr2 inet manual
bridge_ports dummy0
bridge_stp off
bridge_fd 0
pre-up modprobe dummy numdummies=4
and use vmbr2 for your internal only needs for all the nodes that need internal network only. by the way, it will be persistent over reboot...