The present invention relates to an authentication method and system for identifying a subscriber (1) of a first network an address of the second network (9) is allocated to the subscriber (1). An information about a mapping between the address of the second network (9) and a subscriber identity is generated and transmitted to the second network (9). Thereby, an authentication server connection is provided between the first network (2) and the second network (9), such that the subscriber identity can be handled over to the second network (9). Thus, a VAS platform of the second network (9) can receive the address of the second network and the subscriber identity of the subscriber (1), such that subscriber accessing services of the VAS platform can be identified for charging and/or addressing purposes.