#1. Open a terminal window.
#2. Issue the command sudo nano /etc/sysctl.conf
#3. 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

#4. Save and close the file.
#5. Reboot the machine.