Dual Stack
This is the method originally proposed for a smooth transition to IPv6. In this case, enough IPv4 addresses are needed in order to simultaneously deploy both versions of the IP protocol network-wide.
Thus, when a connection is to be established with an IPv4-only host the IPv4 path is used and when a connection is to be established with an IPv6-only host the IPv6 path is used. When the destination can use either protocol, usually IPv6 will be tried first and, if that fails, IPv4 will be used (this behavior has somewhat changed due to the large timeouts involved; see ‘Happy Eyeballs’).