How to disable IPv6 in Ubuntu 14.04?

To disable ipv6, you have to open /etc/sysctl.conf using any text editor and insert the following lines at the end:

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

If ipv6 is still not disabled, then the problem is that sysctl.conf is still not activated.

To solve this, open a terminal(Ctrl+Alt+T) and type the command,

sudo sysctl -p

You will see this in the terminal:

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

After that, if you run:

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

It will report:

1

If you see 1, ipv6 has been successfully disabled.

You might want to disable it right from the boot. For this purpose, open /etc/default/grub with your favorite text editor with root access (maybe sudo vi /etc/default/grub.

In this file, find this line:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

and change it to:

GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 quiet splash"

Save the file and update grub by running:

sudo update-grub

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Mint 19 and Ubuntu 18.04:

on Mint 19 it would be "xed admin:///etc/default/grub" and maybe on Ubuntu 18.04 "gedit admin:///etc/default/grub" to get a GUI editor for doing the edits to the file. The rest of the steps will be the same.