IPV6 is enabled by default in Ubuntu Jaunty 9.04, even if you upgraded from an old version of Ubuntu. So if you want to disable it:

1. Check if you have it enabled or not (just in case) so write this in a terminal:

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

0 = enabled, 1 = disabled - so if you got a 0 in the terminal, then it's enabled and you need to proceed to step 2 to disable it.

2. Write this in a terminal:

sudo su -
echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6

If you do no have that directory, you can edit your grub menu:

nano -w /boot/grub/menu.lst

and add :

ipv6.disable=1

to your kernel.

And you are done, try it again to see if it's disabled (step 1).