Libvirt Network Manual (i)

If you select a network type

On a dedicated server, virtual machines often need to be accessed from the public network (that is, each VM needs a public address), then need to bridge the network, which makes each virtual machine has its own IPV4 and IPV6 address. If the bridge is not available, you can create a routed Network. If the server's IPV4 address is limited, then only the nat-based Network can be selected.

In an enterprise intranet or home network, nat-based Networks enables virtual machines to access the extranet. If a virtual machine runs a service that needs to be accessed by another system, the bridged network is required (Ethernet is required for the host) or Routed network (the host is connected to a wireless network).

Libvirt Network Manual (i)