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I'm not sure if IPv6 in Ubuntu 10.04 Lucid Lynx is still causing issues with the Internet connection being slow and so on, but in case it does here's how to disable it.

To check if IPv6 is disabled, run the following command:
cat /proc/sys/net/ipv6/conf/all/disable_ipv6

0 means it's enabled and 1 - disabled.

This tweak is thanks to an old comment from Stonecut (this way you don't have to edit the GRUB). Basically, to disable IPv6 in Ubuntu 10.04 Lucid Lynx, 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 in the beginning of the post.

If for some reason IPv6 still shows up as enabled in Ubuntu 10.04 Lucid Lynx, you can use the method described in THIS post.

This tweak (and others) will go into Ubuntu 10.04 Start script (I don't know why I didn't think of this before).