By default the ipv6 module is turned on during the installation, you can turn off the module or install/configure the ip6tables tables to prevent your server from attacks.
Steps to disable the ipv6 module.
Edit the file /etc/modprobe.conf in your favorite editor and add the lines below.
alias net-pf-10 off
alias ipv6 off
Edit the /etc/sysconfig/network file
Disable the ipv6 firewall by :
service ip6tables stop
chkconfig ip6tables off
Restart your server and make sure that inet6 addr: is not listing any ipv6 ips in the ifconfig after reboot.