Make the following modifications to disable ipv6 in CentOS (Redhat) Linux:

/etc/modprobe.conf

alias net-pf-10 off
alias ipv6 off

/etc/sysconfig/network

NETWORKING_IPV6=no

/etc/sysconfig/network-scripts/ifcfg-eth0

IPV6INIT=no

Also check the otherl ifcfg-eth0…n and wan0…n scripts.