The Networking Services (XNS), Issue 5.2 specification defines the industry-standard open systems interfaces to communication services, including IPv6. These comprise the Sockets Interface and the X/Open Transport Interface (XTI). The Sockets Interface (part 2 of this document) is now the main industry-standard interface. XTI (part 3 of this document) is now considered to be obsolete, so writers of new applications using the Internet Protocol Suite (IPS) are recommended to use Sockets rather than XTI. Where protocols for which there is no Sockets support are in use, XTI is still recommended in preference to proprietary APIs. XNS Issue 5.2 is a new version of XNS Issue 5 (C523). (Please note, however, that C523 remains the requirement for UNIX 98.) The most important new feature is the inclusion of Internet Protocol version 6 (IPv6) functionality, in a manner which is aligned with the relevant IETF IPv6 standard (RFC 2553).