A packet transmission system for use in a mobile communications network system for routing a packet using an IP address between a first user in the mobile communications network system and a second user, said packet transmission system comprising: a domain-name server including a database for storing IP addresses of users in connection with domain names of the users, characterized by an IP address including a location address of an access link termination node (112; 114) in a subdivided network having established an access link to a user and a user identifier identifying the user in the mobile communications network system; means for receiving from an access link termination node (112; 114) user location registration information of the first user, the location registration information including an IP address of the first user including the location address of the access link termination node (112; 114) and a user identifier identifying the first user in the mobile communications network system; and means for updating the IP address of the first user using the user location registration information received; wherein the access link termination node (112; 114) includes: means for storing the location address of the access link termination node (112, 114); access link management means for establishing or releasing an access link to the first user including storing user location registration information of the first user in a memory in response to a location registration request from the first user transmitted upon establishing the access link and for providing the first user with the location address of the access link termination node (112, 114); means for transmitting the user location registration information of the first user to the domain-name server in response to the location registration request from the first user; and means for routing the packet in accordance with the location address and the user identifier in the IP address. a domain-name server including a database for storing IP addresses of users in connection with domain names of the users, characterized by an IP address including a location address of an access link termination node (112; 114) in a subdivided network having established an access link to a user and a user identifier identifying the user in the mobile communications network system; means for receiving from an access link termination node (112; 114) user location registration information of the first user, the location registration information including an IP address of the first user including the location address of the access link termination node (112; 114) and a user identifier identifying the first user in the mobile communications network system; and means for updating the IP address of the first user using the user location registration information received; wherein the access link termination node (112; 114) includes: means for storing the location address of the access link termination node (112, 114); access link management means for establishing or releasing an access link to the first user including storing user location registration information of the first user in a memory in response to a location registration request from the first user transmitted upon establishing the access link and for providing the first user with the location address of the access link termination node (112, 114); means for transmitting the user location registration information of the first user to the domain-name server in response to the location registration request from the first user; and means for routing the packet in accordance with the location address and the user identifier in the IP address. means for storing the location address of the access link termination node (112, 114); access link management means for establishing or releasing an access link to the first user including storing user location registration information of the first user in a memory in response to a location registration request from the first user transmitted upon establishing the access link and for providing the first user with the location address of the access link termination node (112, 114); means for transmitting the user location registration information of the first user to the domain-name server in response to the location registration request from the first user; and means for routing the packet in accordance with the location address and the user identifier in the IP address.