When the libvirt daemon starts, it creates a default network defined in the /etc/libvirt/qemu/networks/default.xml configuration file. When a new KVM guest is build without specifying any networking options, it will use the default network to communicate with the host OS and other guests and networks. The default libvirt network is using the Network Address Translation (NAT) method. NAT provides a mapping from one IP address space to another, by modifying the IP address in the header of the IP datagram packet. This is especially useful when the host OS provides one IP address allowing multiple guests on the same host to use that address to establish outbound connections. The virtual machines IP addresses are essentially translated to appear as the host machine's IP address.
The default NAT forwarding network defines and sets up a Linux bridge...
