IPv4 socket applications use a AF_INET (equivalent to PF_INET) protocol family. For IPv6, a new protocol family of AF_INET6 (equivalent to PF_INET6) has been defined. The protocol family is the first parameter to the socket() function that is used to obtain a socket descriptor. For most applications, an AF_INET6 socket can be used to communicate with IPv4 and IPv6 clients.