How To Disable The IPv6 on CentOS 6.4

1. Introduction
By default, IPv6 has been set to enabled on linux CentOS 6.4 minimum installation. If you plan to disable this feature, you can follow these steps to completely disable it. This can avoid accidentally loaded IPv6 module. This steps may working on other CentOS version.
2.  Install
Step 1. Create disable-ipv6.conf on /etc/modprobe.d/ and add the following line :

[root@centos64 ~]# vi /etc/modprobe.d/disable-ipv6.conf

Add the following line :

install ipv6 /bin/true

Whenever the system needs to load the ipv6 kernel module, it is forced to execute the command true instead of actually loading the module.
Step 2. Edit /etc/sysconfig/network and modify as below :
Set NETWORKING_IPV6 value to "no" :

NETWORKING=yes
NETWORKING_IPV6=no
HOSTNAME=localhost.localdomain

Step 3. Do not forget to disable and stop the IPv6 firewall if exist :
Disable auto start at boot :

[root@centos64 ~]# /sbin/chkconfig ip6tables off

Stop ip6tables service if running :

[root@centos64 ~]# service ip6tables stop

Step 4. Add "net.ipv6.conf.all.disable_ipv6 = 1" into /etc/sysctl.conf

[root@centos64 ~]# echo "net.ipv6.conf.all.disable_ipv6 = 1" >> /etc/sysctl.conf

or

[root@centos64 ~]# vi /etc/sysctl.conf

Add the following line at the bottom of sysctl.conf file :

..
...
net.ipv4.tcp_syncookies = 1
...

Step 5. Reboot the server is required to complete the steps above. 

Good Luck For You!!