cket={origIPA,origPON) Other ,origPON, destPON,..}} layers origIPA Discrimination origPON, destPON,.. by PON origIPA Discrimination {destIPA, origlPA,PRN,...} by PRN Connectivity services used by IP Fig. 2.10 Discrimination offered by TCP,UDP/IP. dest...=destination... ; IPA=IP address; orig...=originating ; PON=port number; PRN=protocol number TCP offers a connection-oriented service. The TCP correspondence to the OSI CEi is called "socket." A socket is identified by an IP network address (IPA) and a TCP/UDP port number, which implies that users of different sockets can handle different protocols, i.e., they are not instances, as in the CON case of the OSI RM. We call this discrimination by sockets. 2.2.4 Discrimination by Multiple Network Addresses Network addresses (NA) are used inside connectivity layers primarily for routing. That implies that when a message has reached its terminating layer element in the layer (which is a layer element that listens to a particular destination NA), the desti- nation NA has normally ceased to play a role (see the scenario in Fig. 2.4). Thus, destination NAs are normally not used for discrimination (neither in the OSI RM nor in the Internet). In circuit-switching networks, terminating nodes are sometimes assigned multiple NAs. For example, in PSTN and ISDN, terminating nodes (called "end systems") of the "private automatic branch exchange" type (PABX) may support hundreds to thousands of terminals, each having its own ISDN network address. How these are connected to the ISDN is of no concern to the ISDN. Since these NAs are also used for routing inside the ISDN, this is not a discrimination case, but a case of interworking between two separate networks (an ISDN and a PABX) within the same NA numbering system (see Fig. 2.11). NAs within a particular PABX are assigned within a particular series number, which is the part of an NA used for routing inside the ISDN. 2.2 Discrimination (Connectivity Layers Only) control ISDN access These all have protocol control protocol different 