On 11/13/2012 04:05 PM, Patrick Chemla wrote: Never edit the XML files in /etc/libvirt directly - their use is exclusively internal to libvirtd and the results are officially "unpredictable" (in this case, libvirt happens to overwrite the file with its own internal state). Instead, use the libvirt API (or its shell extension, virsh). To modify a network definition, you would use "virsh net-edit $networkname" (virsh uses the editor specified in the EDITOR environment variable, or /usr/bin/vi if EDITOR isn't set.) Once you've edited the network, you will need to restart it. Use: virsh net-destroy $networkname && virsh net-start $networkname Note that any guests connected to that network will need to be restarted (or alternately, their interfaces will need to be detached and re-attached). It's not the IPv6 part that you didn't get, it's the more basic "How do I modify a libvirt network definition". There isn't really much to the IPv6 support on libvirt's networks - you just ad an <ip> element with family='ipv6' (and make the prefix='64' if you want address autoconf to work).