Abstract: A mobile communication system comprising subscribers provided with identifiers (101), one or more mobile exchanges, and a numbering domain allocated to the mobile communication system, the numbering domain comprising subscriber identifiers (101, 103). In the mobile communication system, a predetermined part (104) of the identifier (101, 103) of each subscriber forms an index (104) which refers to a predetermined protocol (106) to be applied to the identifier (103). The protocol (106) consists of one or more rules (108). These rules (108) define variables (111) on the basis of which subscriber identifiers (101, 103) belonging to a predetermined block of the numbering domain are interpreted in a predetermined manner.