For virtual hosts it's necessary to make a bridged network adapter. At least if you want your hosts to be on the same network as the rest of your machines.
Here's how it's done
# The primary network interface auto eth0 iface eth0 inet manual # Bridge adapter auto br0 iface br0 inet static address 192.168.100.4 netmask 255.255.255.0 network 192.168.100.0 broadcast 192.168.100.255 gateway 192.168.100.254 bridge_ports eth0 bridge_stp off bridge_fd 0 bridge_maxwait 0
The primary network adapter needs to be set to auto. allow-hotplug will not work! Also note that the address for the primary network card is actually set in the bridged network section.