IPv6 introduces new APIs for the resolver function. These APIs allow applications to resolve host names to IP addresses and IP addresses to host names. The primary new APIs are getaddrinfo, getnameinfo, and freeaddrinfo. The APIs are designed to work with both IPv4 and IPv6 addressing. Consider use of these new APIs if an application is being designed for eventual use in an IPv6 environment.

How host name (getaddrinfo) or IP address (getnameinfo) resolution is done depends on the resolver specifications that are contained in the resolver setup files and TCPIP.DATA configuration files. These specifications determine whether the APIs query a name server first and then search the local host tables, whether the order is reversed, or even if one of the steps is eliminated. If the APIs have to search local host tables, the specifications also control which tables are accessed. For more information about resolver setup, see Resolver configuration.