Patch for right determination of IPprefix if have more than one network interface.

Hello.
If you have more than one interfaces you have to properly search for IP prefix.
example my ip route output:
default via 192.168.10.1 dev eth1  proto static 
10.8.0.0/24 via 10.8.0.13 dev tun0 
10.8.0.13 dev tun0  proto kernel  scope link  src 10.8.0.14 
192.168.10.0/24 dev eth1  proto kernel  scope link  src 192.168.10.101  metric 9
so IPprefix = ipr[8][2:] is 10.8.0.0/24 but right is 192.168.10.0/24
this patch searching for interface in routes and give right IPprefix