libvirt’s default network configures an isolated bridge device to be used by guest domains. This default bridge creates a private network for the virtual machines, but does not connect that private network to your physical network. The simplest way to complete that connection is to enable IP Forwarding in the kernel. You can quickly enable IP Forwarding using sysctl like so:
# sysctl -w net.ipv4.ip_forward=1
To set your system to enable IP forwarding upon bootup, edit the net.ipv4.ip_forward setting in /etc/sysctl.conf.
While IP forwarding can get you running quickly, for production environments you probably want to configure a more robust bridge network. Information on creating bridge networks for libvirt can be found on the libvirt wiki at: