Centos 6/RHEL disable the IPv6 module.

http://minimallinux.blogspot.com/2013/07/centos-6rhel-disable-ipv6-module.html

IPv6 was introduced to solve the problems of IPv4 but it is often not used and not all hardware supports it. If you find it necessary to disable IPv6 for any reason you may also find that this also speeds up networking and reduces administrative overhead with improved security levels.

IPv6 is a major component found within the Centos operating system and by following this guide you can completely disable IPv6 on the system. For those networks that do not support this feature disabling IPv6 can be a good option in order to tighten system security and increase the overall performance of the system.

It is not advised to use this method if you are intending to use any IPv6 dependent features such as Bonding, Postfix, SELinux and similar packages.

To begin, log in as root and disable IPv6 by typing

# echo "install ipv6 /bin/true" > /etc/modprobe.d/disable-ipv6.conf

This ensures that whenever the system needs to load the IPv6 module, it is forced to execute the true command instead of actually loading the module; and as /bin/true does and means nothing, the module will not load.

Now disable the ipv6 tables

# chkconfig ip6tables off

Then we disable any calls to IPv6 in it's various locations. To do this open the network configuration file.

# vi /etc/sysconfig/network

Scroll down and add or amend the following line to read

NETWORKING_ipV6=no

To complete the process we modify the configuration file for each Ethernet device to show the following values as an example

# vi /etc/sysconfig/network-scripts/ifcfg-ethx  (x = 0,1 etc)

# IPv6INIT=no
# IPv6_AUTOCONF=no

Reboot the system to complete.