My environment: RH FC8 x86_64
  To enable IPv6 globally, modify the file
  # vi /etc/sysconfig/network
  And append the following:
  NETWORKING_IPV6=yes
  Then change the NIC config file
  # vi /etc/sysconfig/network-scripts/ifcfg-eth0
  And append the following IPv6 parameters:
  IPV6INIT=yes     
IPV6ADDR=<IPv6-IP-Address>      
IPV6_DEFAULTGW=<IPv6-IP-Gateway-Address>
  If you want to acquire the IPv6 address via DHCPv6 server, first make sure you have the DHCPv6 client installed:
  # rpm –q dhcpv6-client
  then modifiy the NIC config file and append the following parameters:
  # vi /etc/sysconfig/network-scripts/ifcfg-eth0
  IPV6INIT=yes     
DHCPV6C=yes
  Edit the main DHCPv6 client config file
  # cp /usr/share/doc/dhcpv6-client-*/dhcp6c.conf /etc/     
# vi /etc/dhcp6c.conf
  interface eth0 {      
    send rapid-commit;       
    request domain-name-servers;       
};
  Restart the network service
  # /etc/init.d/network restart
  Then check your eth0 configuration to make sure you get the ipv6 address:
  # ifconfig eth0