Disabling IPv6 may be necessary if you run network services that use IPv6 but have no functional IPv6 networking. For example, using Exim, Postfix or even PHP-FPM may switch to IPv6.
Edit /etc/sysconfig/network
Change the following:
NETWORKING_IPV6=yes to NETWORKING_IPV6=no
Edit /etc/modprobe.conf
Add the following if it does not exist:
alias net-pf-10 off alias ipv6 off
Stop the ipv6tables service
service ip6tables stop
Disable the ipv6tables service
chkconfig ip6tables off
After these changes, IPv6 will be disabled after the next reboot of your system.