The invention relates to a method for automatic roaming between heterogeneous WLANs and/or GSM/GPRS/UMTS networks, in which method, for authentication, a mobile IP node (20) requests access to the WLAN at an access point (21, 22), in which method, upon request from the access server (23), the mobile IP node (20) transmits an IMSI stored on a SIM card (201) of the mobile IP node (20) to the access server (23), and in which method, based on the IMSI, using information stored in an SIM user database (34), the logic IP data channel of the WLAN is supplemented user-specifically towards corresponding GSM data for signal and data channels of a GSM network, and the authentication of the IP nodes (20) is carried out in an HLR (37) and/or VLR (37) of a GSM network.