Assign an IPv6 address to CentOS/Fedora
Delete this file if it exists
rm /etc/modprobe.d/ipv6.conf
If that file existed then you need to reboot your server.
Add this to /etc/sysconfig/network
NETWORKING_IPV6=yes
Add adding these lines to /etc/sysconfig/network-scripts/ifcfg-eth0 (Use your own IPv6 range and gateway)
IPV6INIT=yes
IPV6ADDR=2a04:2e00:5:2a::1::/64
IPV6_DEFAULTGW=2a04:2e00:5:2a::19
Restart the network service.
service network restart