How to enable IPv6 on Centos 6.X

Step 1.
Check IPv6 module is enable for Kernel

lsmod | grep ipv6

step 2.Enable IT.
2.1
##notice may file name is different##
Editing the file /etc/modprobe.d/disable_ipv6.conf
Change
options ipv6 disable=0
to
options ipv6 disable=1

2.2
Editing the file /etc/sysconfig/network
Change
NETWORKING_IPV6=no
to
NETWORKING_IPV6=yes

step 3 rebooting your server

step 4
configure kernel parameters to enable IPv6

sysctl -w net.ipv6.conf.all.disable_ipv6=0
then reboot

step 5
Checking if it works.

sysctl -a | grep ipv6

then it will outputting something like net.ipv6.confXXXXXXX = 0.

all done.