Hello,
I want to make the default network assign ips in the range of 192.168.122.101 - 192.168.122.254 instead than default values.

From virt-manager it seems that the setting is not editable.
Changing the file /etc/libvirt/qemu/networks/default.xml and restarting libvirtd and/or the system it seems the file actually is not parsed as I see:
[root@tekkaman ~]# ps -ef|grep dns
nobody    1464     1  0 18:59 ?        00:00:00 /usr/sbin/dnsmasq --strict-order --bind-interfaces --pid-file=/var/run/libvirt/network/default.pid --conf-file=  --listen-address 192.168.122.1 --except-interface lo --dhcp-range 192.168.122.2,192.168.122.254 --dhcp-lease-max=253

and the same range in virt-manager networks window....

Is there a method to change behaviour so that in default virbr0 I have s subset of fixed ip and a subset of dhcp assigned ones?

Using F12 + rawvirt repository
libvirt-0.7.6-1.fc12.x86_64

Thanks,
Gianluca