What do you mean by numbering and addressing?

Numbering and Addressing: In data networks and telephone, the end equipments are more frequently single units than multiple devices units as PABX or LAN. In the past, a telephone, a terminal or a computer has been the predominant end equipment. For these networks the numbering system has also evolved to specify single equipment end points. In ISDN, multiple devices on the end points are more of a norm than single units, in analysis of the multiple facilities environment. This then becomes essential to identify specific end equipment, for example: facsimile or computer, to render the service. Identifying the specific equipments in a two-level process; first the end point is identified as in the case of telephone or data networks and after that the equipment at the end point. ISDN addressing structure gives for this requirement. The element of the ISDN address that is used to specify the end points is termed as the ISDN number, and the element for identifying the specific equipment on the end point is called the ISDN sub address.