Guide on how to disable ipv6 for Centos and RedHat
1) Edit /etc/sysconfig/network-scripts/ifcfg-eth0 and change:
NETWORKING_IPV6=yes to NETWORKING_IPV6=no
2) Edit /etc/modprobe.conf and add these lines:
alias net-pf-10 off
alias ipv6 off
3) Stop the ipv6tables service:
service ip6tables stop
4) Disable the ipv6tables service:
chkconfig ip6tables off
Finally, reboot of your system.