Add the code below to /etc/sysctl.conf
sudo vi /etc/sysctl.conf
# IPv6 net.ipv6.conf.all.disable_ipv6 = 1 net.ipv6.conf.default.disable_ipv6 = 1 net.ipv6.conf.lo.disable_ipv6 = 1
sudo sysctl -p
links
http://www.noobslab.com/2012/05/disable-ipv6-if-your-internet-is.html