ITU-T I.334 PDF

Introduction

Recommendation X.200, covering the open systems reference model, applies the term "address" to identify service access points at each layer. With respect to the network layer, a service access point may be identified by an ISDN number/subaddress. This Recommendation is provided to clarify the concepts and terminology which relate ISDN numbers and subaddresses to one another and to OSI reference model network layer addresses.