Comment # 39 on bug 1158817 from James Fehlig

(In reply to Neil Rickert from comment #38)
> Looking closely -- yes, it is there.  It seems to be defining a NAT network
> with IP addresses of the form 192.168.122.xxx.  I have never seen such an
> address on my network.

Likely because none of your VMs use that network.

> My virtual machines get addresses of the form
> 192.168.1.xxx, and those come from the home router.  So I don't think I am
> actually using "virbr0".

If you are not using it, you should disable it. E.g. 'virsh net-destroy default
&& virsh net-autostart default --disable'.

> Wouldn't I need to start some kind of NAT service
> before that would be available?

libvirt starts an instance of dnsmasq to provide dhcp and such services for VMs
using the virtual network.