This is an ubuntu/debian recipe to use an 'unnumbered bridge' to save on the amount of IP addresses needed to connect your virtual machines on a libvirt host to other networks.
I'm assuming you want the host to be a router between the VM's and the external network. The advantage of this is that you can firewall traffic between the virtual machines and other networks on the host.
A setup like this can be used if your ISP provides you with a /29 and you want to be able to use every IP address out of that /29, and not waste IP's on the network, broadcast and gateway address.
This image shows the various elements involved:
The /etc/network/interfaces file on the host:
iface virbr0 inet manual
post-up ip route add 10.10.10.0/29 dev virbr0
The above configuration will configure a bridge interface without an IPv4 address and route the /29 that was assigned to you by your ISP to that interface. This will force Linux to ARP for every IP from that /29 on this particular virbr0 interface.
The following virsh commands will remove the default network settings, you can Continue reading