Moving exclusively to IPv6 is not a practical option for most organizations. Therefore, it is possible to make the move gradually and to use mixed environments while the transition is taking place. The question for most organizations is when, how, where, and how much to transition.
Some individual networks within an organization can be upgraded as a whole, creating small IPv6 networks surrounded by IPv4 networks, but IPv4/IPv6 gateways are necessary at the borders of these networks to interoperate with IPv4 networks. Different IPv6 networks can also communicate with each other through the IPv4 Internet by setting up IPv6/IPv4 tunnels.
Some organizations will migrate host by host, with dual-protocol IPv4/IPv6 nodes scattered throughout the existing IPv4 network. These nodes can interoperate with each other in native IPv6, or with IPv6 nodes outside the network by tunneling IPv6 inside IPv4 packets.
See the following sections for more information about how IPv6 interoperability with IPv4 is enabled: