Both Red Hat Enterprise Linux 4 and 5 enable Internet Protocol Version 6 (IPv6) by default. Some users do not find it worth using, if that’s the case with you then you can always disable the same.

How to disable:
======================
To disable IPv6 support in RHEL 4, remove the following line, if it exists, from the /etc/modprobe.conf file.

alias net-pf-10 ipv6

and instead, add:

alias net-pf-10 off

to make sure that this module will not get loaded from now on. Reboot your RHEL 4 system now to complete the process.

However, if using RHEL 5, also add the following line to the /etc/modprobe.conf file:

alias ipv6 off

It is also a good practice to change the NETWORKING_IPV6 line in the /etc/sysconfig/network file to the following:

NETWORKING_IPV6=no

and also make sure that ip6tables does not start from the next reboot.

chkconfig ip6tables off

Now, reboot the RHEL 5 system to disable IPv6 support.

How to re-enable:
======================
Just undo the changes you made above!