Add the following at final of your file '/etc/sysctl.conf':
net.ipv6.conf.all.disable_ipv6 = 0
net.ipv6.conf.default.disable_ipv6 = 0
net.ipv6.conf.lo.disable_ipv6 = 0
And execute:
# sysctl -p
But *BEFORE* check you firewall is configured and enabled to catch ipv6 events.
Regards,
Netino