To disable IPv6 permanently :
open editor with
sudo nano /etc/sysctl.conf
Add the following at the bottom of the file:
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
In case to change it only temporarily use:
sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1
sudo sysctl -w net.ipv6.conf.default.disable_ipv6=1
Replace 1 with 90 to enable it again