By default Ubuntu comes with IPv6 enabled. When you don't have an IPv6 Internet connection or you want to disable IPv6 from a security perspective, it's easy to disable:
1. sudo su
2. echo "net.ipv6.conf.all.disable_ipv6 = 1" >> /etc/sysctl.conf
3. option 1: reboot
3. option 2: sysctl -p