A communication system comprising: a transfer node adapted to transmit a registration request signal for requesting registration of an address of a mobile node; and a server adapted to receive the registration request signal from the transfer node, wherein the transfer node comprises a control section adapted to, when a transmission trigger for the registration request signal occurs, determine the type of the transmission trigger, and depending on the determined type, determine whether or not the number of mobile nodes which will be registered or which will have a registration expiry time postponed are a plurality, in accordance with the registration request signal, wherein if the plurality of mobile nodes are not to be processed, the transfer node generates and outputs a registration request signal intended for one mobile node, to the server, and if the plurality of mobile nodes are to be processed, the transfer node generates and transmits, to the server, a piece of information indicating that the plurality of mobile nodes will be processed, or a piece of information indicating the plurality of mobile nodes which will be processed, or an aggregation registration request signal with both pieces of information, wherein the server is adapted to, upon receiving the registration request signal from the transfer node, to execute a process of registering the mobile node specified in the registration request signal or of postponing the registration expiry time for the mobile node, and upon receiving the aggregation registration request signal, to execute a process of registering the specified plurality of mobile nodes or of postponing the registration expiry time for the specified plurality of mobile nodes, wherein the transfer node is further adapted to, upon receiving the registration request signal from the mobile node, determine whether or not the registration request signal is to be an aggregation target, depending on the type of the registration request signal, and if a determination result indicates that the registration request signal is not to be an aggregation target, to transmit the registration request signal to the server, and if the determination result indicates that the registration request signal is to be an aggregation target, to hold the registration request signal until a predetermined time, aggregate held registration request signals to generate the aggregation registration request signal, and transmit the aggregation registration request signal to the server at the predetermined time. a transfer node adapted to transmit a registration request signal for requesting registration of an address of a mobile node; and a server adapted to receive the registration request signal from the transfer node, wherein the transfer node comprises a control section adapted to, when a transmission trigger for the registration request signal occurs, determine the type of the transmission trigger, and depending on the determined type, determine whether or not the number of mobile nodes which will be registered or which will have a registration expiry time postponed are a plurality, in accordance with the registration request signal, wherein if the plurality of mobile nodes are not to be processed, the transfer node generates and outputs a registration request signal intended for one mobile node, to the server, and if the plurality of mobile nodes are to be processed, the transfer node generates and transmits, to the server, a piece of information indicating that the plurality of mobile nodes will be processed, or a piece of information indicating the plurality of mobile nodes which will be processed, or an aggregation registration request signal with both pieces of information, wherein the server is adapted to, upon receiving the registration request signal from the transfer node, to execute a process of registering the mobile node specified in the registration request signal or of postponing the registration expiry time for the mobile node, and upon receiving the aggregation registration request signal, to execute a process of registering the specified plurality of mobile nodes or of postponing the registration expiry time for the specified plurality of mobile nodes, wherein