Disable IPv6 on Ubuntu

http://www.noobslab.com/2012/05/disable-ipv6-if-your-internet-is.html

add to /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

then

sysctl -p