In general, IPv6 connectivity between two hosts is preferred over IPv4 connectivity. In many cases, IPv4 is used only if one of the nodes does not support IPv6. This can lead to undesirable paths in the network being used for communication between two hosts. For instance, when a native IPv6 path does not exist, data can be tunneled over the IPv4 network, even when a native IPv4 path exists.

This can lead to longer connection establishment to an AF_INET application which resides on a dual-stack host. The client first attempts to connect using each IPv6 address defined for the dual-stack host before attempting to connect with IPv4. A well-behaved client cycles through all the addresses returned and ultimately, connects using IPv4. However, this takes both time and network resources to accomplish, and not all clients are well-behaved or bug-free.

To avoid undesirable tunneling and other potential problems, configure two host names in DNS. The existing host name can continue to be used for IPv4 connectivity to minimize disruption when connecting to unmodified AF_INET server applications. A new host name can also be defined, for which both IPv4 and IPv6 can be configured. When connecting by using the old host name, AF_INET6 clients connect by using IPv4. When connecting by using the new host name, AF_INET6 clients attempt to connect by using IPv6 and, if that fails, fall back and connect by using IPv4.

Using two host names allows the client to choose the network path that is taken. The client can route over IPv6 when the destination application is IPv6 enabled and a native IPv6 path exists, or take an IPv4 path.

The use of distinct host names for IPv4 and IPv4/IPv6 addresses is not strictly required. A single host name can be used to resolve to both IPv4 and IPv6 addresses. In addition, the use of distinct host names is necessary only during the initial transition phase when native IPv6 connectivity does not exist and applications have not yet been enabled for IPv6. After both of these occur, a single host name can be used.