Apparently your model of modem/router does not support IPv6 (see http://www.dslreports.com/forum/r209684 ... -next-step).
See the SL Forums thread Possible IPv6 Issue [Solved]. Try the modification to /etc/modules.d/aliases given in that thread. If that does not work then try the other work-around given in that thread. If you cannot do that then you would either have to rebuild the kernel to disable IPv6 support or change your modem to a more modern version.
