The present invention relates to a method for application layer authentication of subscribers connected to the authenticating network domain by a 2G or 2.5G GPRS core network or a 3G UMTS network, characterised by using data which are assembled by the network layer during establishment of a PDP context in GPRS networks. The invention comprises System of units in a mobile telecommunication network, including at least a first authentication unit which is connected via a data line to a second unit which assembles data according to the described method.