I. CentOS 5
-Edit /etc/sysconfig/network
# vi /etc/sysconfig/network

NETWORKING_IPV6=no

-Edit /etc/modprobe.conf
# vi /etc/modprobe.conf

alias net-pf-10 off
alias ipv6 off
options ipv6 disable=1

-Matikan ip6 firewall dan disable service ip6tables:
# service ip6tables stop
# chkconfig ip6tables off

-Restart mesin:
# reboot

 
II. CentOS 6
-Edit /etc/sysctl.conf
# vi /etc/sysctl.conf

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

-Matikan ip6 firewall dan disable service ip6tables:
# service ip6tables stop
# chkconfig ip6tables off

-Reboot machine
# reboot

 
References:
http://wiki.centos.org/FAQ/CentOS5#head-47912ebdae3b5ac10ff76053ef057c366b421dc4
http://wiki.centos.org/FAQ/CentOS6#head-d47139912868bcb9d754441ecb6a8a10d41781df