Disable IPv6 on CentOS 5.2 and Fedora 9

vi /etc/sysconfig/network

set "NETWORKING_IPV6" to "no"

vi /etc/modprobe.conf

Add

alias ipv6 off
alias net-pf-10 off

Run chkconfig to disable the IPv6 firewall
chkconfig --level 345 ip6tables off

Reboot the system