8

First, edit /etc/default/grub and find the line:

GRUB_CMDLINE_LINUX=""

and change the line to say this instead (this will disable ipv6 completely):

GRUB_CMDLINE_LINUX="ipv6.disable=1"

alternatively, to leave the ipv6 stack functional but to disable assignment of
ipv6 addresses you can use the following option instead:

GRUB_CMDLINE_LINUX="ipv6.disable_ipv6=1"

Finally, run:

sudo update-grub

and reboot to apply the changes.

This will disable ipv6 at the kernel level so that it is never enabled from the
get-go.

Also, after making the following changes to /etc/sysctl.conf

net.ipv6.conf.eth0.disable_ipv6 = 1

Run the following command to apply the changes:

sudo sysctl -p

Finally, if using the option to disable ipv6 in sysctl.conf, you need to also
make sure ipv6 is commented out in /etc/hosts. See here
https://wiki.archlinux.org/index.php/IPv6#Disable_functionality