A method and a system for allowing a user device that has already been authenticated by a first communications network to gain access to a second communications network without undergoing authentication by the second communications network. The first communications network and the second communications network have a pre-established trust relationship there between. A packet is received from the user device that includes a user device public key, by the second network via the first network. A session key is sent from the second network to the user device, via the first network, when a source Internet Protocol (IP) address associated with the packet falls into a range allocated to the first network. The session key is encrypted with the user device public key. The user device decrypts the session key using a private key and uses the session key thereafter to access the second network. Further a mapping is generated to correlate the identity of the user device with the session key such that usage data relate to user device is generated by the second communications network and transmitted to the first communications network, which generates accounting information indicative of user device access of the second communications network.