1. Open the terminal and issue this command to check whether IPv6 is enabled or not:

cat /proc/sys/net/ipv6/conf/all/disable_ipv6

0: means enabled

1: means disabled

2. To disable IPv6, we need to edit the ‘sysctl.conf‘ file. So, via the terminal, issue this command:

gksudo gedit /etc/sysctl.conf

Add the following lines in the end of file

# IPv6
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

Or you can also

Add ipv6.disable=1 to /etc/default/grub

GRUB_CMDLINE_LINUX_DEFAULT=”ipv6.disable=1″
And then you can run update-grub.