The defacto standard Application Program Interface （API） for TCP/IP applications is the “sockets” interface. Although this API was developed for （  ） in the early 1980s it has also been implemented on a wide variety of non-Unix systems。 TCP/IP （  ）written using the sockets API have in the past enjoyed a high degree of portability and we would like the same （  ）with IPv6 applications。 But changes are required to the sockets API to support IPv6 and this memo describes these changes。These include a new socket address structure to carry IPv6 （  ），new  address conversion functions，and some new socket options。These extensions are designed to provide access to the basic IPv6 features required by TCP and UDP applications，including multicasting， while introducing a minimum of change into the system and providing complete （  ）for existing IPv4 applications。