Possibly never. The purpose of deploying IPv6 is to ensure network growth and continued interconnectivity when IPv4 address space becomes depleted and DIFFICULT to obtain. In addition, as the global Internet continues to expand, it is likely that some Internet sites will only be available via IPv6.

To avoid problems, one should be fully IPv6-enabled by the time IPv6-only sites START appearing. However, in practice, it is only the public (or user) facing part of an enterprise's infrastructure that needs to be IPv6 enabled at the outset. The back-end infrastructure - which users do not interact with directly - can continue to be BASED entirely on IPv4, so long as that is the most cost-effective approach. (Enterprises may determine that it is more cost-effective to PROGRESSIVELY turn off IPv4 in PARTS of their network once it is no longer needed or in significant use.)

One should expect, however, that it might never be cost-effective (or possible) to upgrade certain legacy systems. Thus, it will likely be a decade or more before enterprise sites find themselves in a position to consider completely turning off IPv4. In practice, there is no need to turn it off so long as IPv4-only applications still remain in use.

Possibly never. The purpose of deploying IPv6 is to ensure network growth and continued interconnectivity when IPv4 address space becomes depleted and difficult to obtain. In addition, as the global Internet continues to expand, it is likely that some Internet sites will only be available via IPv6.

To avoid problems, one should be fully IPv6-enabled by the time IPv6-only sites start appearing. However, in practice, it is only the public (or user) facing part of an enterprise's infrastructure that needs to be IPv6 enabled at the outset. The back-end infrastructure - which users do not interact with directly - can continue to be based entirely on IPv4, so long as that is the most cost-effective approach. (Enterprises may determine that it is more cost-effective to progressively turn off IPv4 in parts of their network once it is no longer needed or in significant use.)

One should expect, however, that it might never be cost-effective (or possible) to upgrade certain legacy systems. Thus, it will likely be a decade or more before enterprise sites find themselves in a position to consider completely turning off IPv4. In practice, there is no need to turn it off so long as IPv4-only applications still remain in use.