Disable ipv6 in Ubuntu

1. sudo gedit /etc/modprobe.d/aliases
2. Find the line: alias net-pf-10 ipv6
3. Edit this to: alias net-pf-10 off
4. Save the file and reboot
5. Check ipv6. There should not be any output to any of the following commands.
   ip a | grep inet6
   lsmod | grep v6
6. change network.dns.disableIPv6 to true in about:config of Firefox.
   

[ Reference(s): Ubuntuforums ]