od which implies that routes between hosted nodes are assigned the same SI. Figure 3.28 demonstrates how the distinction between layer APi (applied by MTP L3) and layer-element APi (applied by SCCP) affects operations: The SCCP user must deliver as address parameters both the MTP L3 network address (DPC) and a called SSN. SCCP gets the calling SSN from its T-AP table. SCCP must include both the called and the calling SSN in its messages. MTP L3 requires only the DPC as address parameter, since the called SI is the same as the calling SI. The actual SI for SCCP is stored in the MTP L3 T-AP table. Thus, by adding SCCP with its subsystem numbers, one introduces two levels of APi. The address parameters that the SCCP user uses for referring a particular route is a destination network address and an APi. We call such routes socket routes (using a terminology introduced by TCP/IP). The model in Fig. 3.28 shows that these parameters belong to different layers, which actually contradicts the OSI RM definition of layers. We do not mind, however, since the SCCP user is not bothered by that. Since SS7 was never intended as a packet-switching network for public use, the creators of SCCP most likely found an SSN range of 256 (eight bits) to be more than enough for future needs. They seem to have been right, since today only 10 numbers are assigned. As a matter of fact, most of these are not even used as APi, but as a substitute to the absence of actor-layer identifiers (ALi) in TCAP (a com- mon agent layer that runs on SCCP), as is discussed in Sect. 6.3. The definition and use of an APi parameter in SCCP cannot be compared to how the corresponding APi, the port, is used in the IP stratum. The purpose with IP was never to support applications that run directly on IP, but to support different kinds of management layers, as well as layers that could adapt the raw, unreliable data- gram service to connectivity services that were better suited to application net- works, especially TCP (protocol number 6) and UDP (protocol number 