Method for registering a communication terminal

A method for registering a communication terminal with a service network which organizes a communications service, where the service network is accessed by the communication terminal using an access network which connects the communication terminal to the service network, in which

• in a preliminary step, a logon IP address which was assigned to the communication terminal when the communication terminal logged onto the access network is received by the service network and is stored therein for the respective user of the communication terminal,
• when a first registration message sent by the communication terminal is received a token is transmitted to the communication terminal using a transport message sent using the logon IP address, the token having been produced by the service network from the logon IP address and a random number using an encryption method, and token data associated with the token having been stored in the service network,
• when a second registration message which is sent by the communication terminal and contains the token is received the service network compares the token received with the token data,
• successful registration is identified if the token matches the token data, and
• unsuccessful registration is identified if the token does not match the token data.