Ubuntu has a newer Internet protocol called IPv6 turned on by default. However, some hardware — such as NICs and modems — shows broken behavior when exposed to IPv6 related DNS requests.

On some connections, the ipv6 kernel module may cause significant slowdowns and fail to connect to IPv6 servers.

To check if IPv6 is Enabled/Disabled, run the following command in terminal.

cat /proc/sys/net/ipv6/conf/all/disable_ipv6
0 means Enabled
and
1 means Disabled

To disable IPv6 in Ubuntu, you have to add the following lines to /etc/sysctl.conf:

#disable ipv6
net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
To do it from the command line, paste this in a terminal:

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
Then reboot and check if IPv6 has been disabled with the command

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