These instructions are for using the FlatDHCP networking mode with a single network interface. More complex configurations are described in the networking section, but this configuration is known to work. First, setup your /etc/network/interfaces file with these settings: eth0: public IP, gateway br100: no ports, stp off, fd 0, first address from --fixed_range set in nova.conf files. Here's an example: # The loopback network interface auto lo iface lo inet loopback # The primary network interface auto eth0 iface eth0 inet dhcp # Bridge network interface for VM networks auto br100 iface br100 inet static address 192.168.100.1 netmask 255.255.255.0 bridge_stp off bridge_fd 0 Also install bridge-utils: sudo apt-get install bridge-utils Ensure that you set up the bridge, although if you use -\-flat_network_bridge=br100 in your nova.conf file, nova will set up the bridge for you when you run the nova-manage network command. sudo brctl addbr br100 Lastly, restart networking to have these changes take effect. (This method is deprecated but "restart networking" doesn't always work.) sudo /etc/init.d/networking restart