Levels of Addressing

X.25 allows call addressing to use an interface's programmatic access name, the X.121 address and subaddress, and protocol ID. This information is contained in the x25addrstr structure, described below.

The application can specify which X.25 interface to use when receiving and connecting calls. This level of addressing is only useful when there is more than one X.25 interface (as with dual-port cards or systems with multiple cards) connected to the HP 9000 system.

	NOTE: For the purposes of this discussion, and throughout this book, you should understand the distinction between the following terms: Card—refers to physical communications hardware, Interface (or Port)—used interchangeably to designate the physical point of connection for communications, and Device—a logical entity (internal to the communications software) that is logically associated with a particular interface.

The application can specify which X.121 address to use when the connection is established. Each interface connected to an X.25 PSN is assigned a unique X.121 address. When an interface is connected to a PSN, the subaddress also designates the X.25 interface (although for test purposes in back-to-back configurations, it is possible to use a different X.121 address than that specified for the PSN interface at configuration time).

The application can specify the X.121 subaddress to use in connecting the call. The subaddress may be used to select a particular type of application on the other end of the call.

The applications may also use the protocol ID to further select the type of application on the other end of the call. Protocol ID addressing is fully described in the CCITT X.244 (1984) Recommendations or chapter 6 of the X.25 (1980) Recommendations.