You need to be very careful not to hit a valid use case here. RFC3069 provides for a mechanism whereby a supernet/subnet system is setup to allow better use of available IPv4 address space. This is becoming of increasing concern as IPv4 space exhausts. To implement this you turn on Proxy ARP on a host and inject /32 routes advertised from other hosts into the routing table. The network on the current host has a wide subnet mask (for example 10.0.0.0/8), so that the host responds to ARPs for VM guests that are actually on a completely different host. So you could have the situation where you have routes picked up via a dynamic routing protocol (say OSPF) on a host that are inside the range for the defined network in libvirt.  Failing to start the network in that case would be incorrect. -- Neil Wilson -- Neil Wilson