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
service ip6tables stopchkconfig ip6tables off