Disable IPv6 on Ubuntu

Edit your /etc/sysctl.conf file and add the following to the bottom:

#disable 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 use the following script:

echo "#disable ipv6" | sudo tee -a /etc/sysctl.conf
echo "net.ipv6.conf.all.disable_ipv6 = 1" | sudo tee -a /etc/sysctl.conf
echo "net.ipv6.conf.default.disable_ipv6 = 1" | sudo tee -a /etc/sysctl.conf
echo "net.ipv6.conf.lo.disable_ipv6 = 1" | sudo tee -a /etc/sysctl.conf

For this changes to take effect you must reboot your system.
After rebooting you can check if IPv6 has been disabled with the following command:

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

0 means it's enabled and 1 - disabled.