PATENT DOCUMENT

Abstract:
A mobile node moves from a first IP (Internet Protocol) network to a second IP network in a network system in which the first IP network capable of executing communication in accordance with both first and second kinds of IPs and the second IP network capable of executing communication in accordance with only the first kind of IP are connected with each other. When the mobile node communicates a message with other nodes on the first network after its movement accordance with the second kind of IP, a header for the movement containing both home and foreign addresses of the first kind in IP is added to a header containing home and foreign addresses in the second kind of IP, and put to the message, is added. The message to which the movement header is thus added is used for the communication between a first mobile agent on the first network and a second mobile agent on the second network, or between the mobile node and the first mobile agent.

Full Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a mobile node, a mobile agent and a network system. More particularly, this invention relates to a control method which assists the movement of a node between an IP (Internet Protocol) network capable of executing communication in accordance with both IP version 4 and an IP version 6 and an IP network capable of executing communication in accordance with only the IP version 4 or an IP network capable of executing communication in accordance with only the IP version 6, a mobile agent, and a network system for assisting the movement of the node. 
     With a drastic development of small and lightweight nodes and the Internet, the demand for taking out a node from an office or a home to utilize it everywhere has been increased. When the node is moved to other network in the conventional network environment making use of the TCP/IP (Transmission Control Protocol/Internet Protocol), however, setting of the IP address, which is the information for primarily identifying the node in the IP network, must be changed so as to match with the foreign or visiting network environment. 
     Even if this change of setting of the IP address is automatically made by utilizing a DHCP (Dynamic Host Configuration Protocol) described in RFC (Request For Comment) 1541 as one of the methods of distributing automatically the IP addresses, there remains the problem that the network connection that has been established already with other nodes by using the IP addresses used in the network before the movement cannot be maintained in succession. 
     Therefore, methods of assisting the movement of the node between the networks have been devised. A typical among them is a protocol of the third layer (network layer) of an OSI (Open Systems Interconnection) reference model and this protocol pertains to the IP version 4 (hereinafter called the “IPv4”) that has gained a wide application in the Internet and the IP version 6 (hereinafter called the “IPv6”) the specification of which has now been stipulated so as to solve the problems of address exhaustion in the IPv4. As to these IPv4 and IPv6, “IP Mobility Support in IPv4”) (hereinafter called “Mobile IPv4”) described in RFC2002 and “Mobility Support in IPv6”) (hereinafter called “Mobile IPv6”) described in IETF (Internet Engineering Task Force) draft (the latest version of which is “draft-ietf-mobile-ip-ipv6-02.txt”) are examples of the known references. 
     Incidentally, the term “IPv4” used in this specification designates an IP address having an address length of 32 bits while the term “IPv6” designates an IP address having an address length greater than 32 bits. 
     By making use of these Mobile IPv4 and Mobile IPv6, a user can execute communication in the same way before the movement of the node even when the node is moved to another network, without the necessity for changing the IP address of the node or cutting off the network connection that has already been established with other node before the movement. 
     Incidentally, the term “node” used in this specification designates all those devices which have an IP address and execute communication by utilizing the IP, such as a PC (Personal Computer), a WS (Work Station), a router, and so forth. 
     Generally, it is assumed that the movement from the IPv4 to the IPv6 is effected gradually and all the networks do not utilize at once the IPv6. In the mean time, therefore, there exist a network (hereinafter called the “IPv4 network”) comprising only those nodes which execute communication by utilizing only the IPv4 (hereinafter called the “IPv4 nodes”), a network (hereinafter called the “IPv6 network”) comprising only those nodes which execute communication by utilizing only the IPv6 (hereinafter called the “IPv6 node”) and a network (hereinafter called the “IPv4/v6 network”) comprising those nodes which execute communication by utilizing both of IPv4 and IPv6 in mixture (hereinafter called the “IPv4/v6 node”), the IPv4 nodes and the Ipv6 nodes. 
     To beginning with, let&#39;s consider the case where the IPv4/v6 network is the one that supports both of Mobile IPv4 and Mobile IPv6. In the Mobile IPv4, messages are exchanged between a mobile node moving between the networks and a mobile agent (hereinafter called the “IPv4 mobile agent”) for assisting the movement of the mobile node which executes communication by utilizing the IPv4, in accordance with the Mobile IPv4 procedures. Similarly, in the Mobile IPv6, messages are exchanged between a mobile node moving between the networks and a mobile agent (hereinafter called the “IPv6 mobile agent”) for assisting the movement of the mobile node that executes communication by utilizing the IPv6, in accordance with the Mobile IPv6 procedures. 
     Let&#39;s consider the case where the IPv4/v6 mobile node supporting both of Mobile IPv4 and Mobile IPv6 inside the IPv4/v6 network moves to another IPv4/v6 network. Because the foreign IPv4/v6 network can execute communication by utilizing both of IPv4 and IPv6, the IPv4/v6 mobile node can exchange the messages with both of the IPv4 mobile agent and the IPv6 mobile agent on the network in accordance with the procedures of the Mobile IPv4 and the Mobile IPv6. Therefore, the movement of this IPv4/v6 mobile node between the networks is supported by both of the Mobile IPv4 and the Mobile IPv6. In consequence, the IPv4/v6 mobile node that has moved to the foreign network can successively execute communication without changing setting of the IP address and without cutting off the network connection that has been established already with other IPv4 node or the IPv6 node before its movement by utilizing the IPv4 or IPv6. It can also execute afresh communication with other node by utilizing the IPv4 and the IPv6. 
     Next, let&#39;s consider the case where the IPv4/v6 mobile node moves from the IPv4/v6 network to the IPv4 network which can execute communication in accordance with only the IPv4 and supports the Mobile IPv4. In this case, since communication by utilizing the IPv4 is possible between the IPv4/v6 mobile node and the IPv4 mobile agent, the assistance of movement of this mobile node between the networks by the Mobile IPv4 can be made. Therefore, the IPv4/v6 mobile node can execute communication successively after the movement without cutting off the network connection that has been previously established already with other IPv4 node by utilizing the IPv4. The mobile node can also execute communication afresh by utilizing the IPv4. 
     However, the mobile node cannot execute communication by utilizing the IPv6 on the IPv4 network and consequently, the exchange of the message on the IPv4 network in accordance with the Mobile IPv6 procedure becomes impossible between the IPv4/v6 mobile node and the IPv6 mobile agent. In other words, the assistance of the movement of the mobile node to the IPv4 network in accordance with the Mobile IPv6 becomes impossible and the IPv4/v6 mobile node that has moved to the IPv4 network cannot maintain the network that has been established already with other IPv6 node by utilizing the IPv6 before the movement and consequently, cannot execute communication. This mobile node cannot execute afresh communication with other node on the IPv4 network by utilizing the IPv6, either. 
     Similarly, let&#39;s consider the case where the IPv4/v6 mobile node moves from the IPv4/v6 network to the IPv6 network which can execute communication by utilizing only the IPv6 and supports the Mobile IPv6. In this case, too, the IPv4/v6 mobile node cannot execute communication by utilizing the IPv4 on the IPv6 network. In consequence, the exchange of the message in accordance with the Mobile IPv4 procedure is not possible on the IPv6 network between the IPv4/v6 mobile agent and the IPv4 mobile agent, so that the assistance of the movement of this mobile node to the IPv6 network in accordance with the Mobile IPv4 becomes impossible on the IPv6 network. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a mobile node, a mobile agent and a network system which can successively maintain the network connection the IPv6 that has been established already by utilizing the IPv6 before the movement when the IPv4/v6 mobile node moves from the IPv4/v6 network to the IPv4 network, and which can also execute afresh communication by utilizing the IPv6. 
     It is another object of the present invention to provide a control method of a mobile node, a mobile agent and a network system for assisting the movement, which can execute communication by utilizing the IPv4 between an IPv4/v6 mobile node and other IPv4 node even when the IPv4/v6 mobile node moves from an IPv4/v6 network to an IPv6 network, without changing at all existing IPv6 mobile agents and existing IPv4/v6 mobile agents and without changing setting of the address of the IPv4/v6 mobile node. 
     According to one aspect of the present invention, there is provided a mobile node including IPv4 (Internet Protocol version 4) processing means for executing services in accordance with the IPv4, IPv6 (Internet Protocol version 6) processing means for executing services in accordance with the IPv6, and communication processing means for executing transmission/reception control of packets to and from networks, and moving between IP networks, wherein the mobile node further comprises movement registration processing means for adding an IPv4 header (IP header used for the IPv4), in which the IPv4 address of a mobile agent is set as a foreign address and the IPv4 address of the mobile node usable in a foreign IPv4 network is set as a home address, to a message used for the IPv6 for registering the movement to a mobile agent connected to the IPv4/v6 network to assist the movement of the mobile node, and transmitting the message, when this mobile node moves from the IPv4/v6 network (a network capable of executing communication by utilizing both of the IPv4 and the IPv6) to an IPv4 network (a network capable of executing communication by utilizing only the IPv4). 
     In the mobile node according to the aspect of the invention described above, the IPv4 header is added to the message used for the IPv6 and the message is then transmitted. Therefore, the message to be used for the IPv6 can be substantially transmitted from the foreign IPv4 network, and the information necessary for the network connection utilizing the IPv6 can be registered to the mobile agent. 
     According to another aspect of the present invention, there is provided a mobile agent including IPv4 processing means for executing services in accordance with an IPv4, IPv6 processing means for executing services in accordance with an IPv6 and communication processing means for executing transmission/reception control of packets to and from networks, and moving between the networks, wherein the mobile agent further comprises packet transmission processing means for generating an IPv4 encapsulated IPv6 packet by adding an IPv4 header, in which the IPv4 address of the mobile agent is set as a foreign address and the IPv4 address of a mobile node usable in a foreign IPv4 network is set as a home address, to an IPv6 packet (packet used for the IPv6) to be transmitted to other node, and transmitting the IPv4 encapsulated IPv6 packet so generated. 
     In the mobile agent according to the aspect of the invention described above, after the IPv4 header is added to the IPv6 packet, the packet is transmitted. Therefore, the IPv6 packet can be transmitted substantially from the foreign IPv4 network. 
     According to still another aspect of the present invention, there is provided a mobile node including IPv4 processing means for executing services in accordance with the IPv4, IPv6 processing means for executing services in accordance with the IPv6 and communication processing means for executing transmission/reception control of packets to and from networks, and moving between the networks, wherein the mobile node further comprises movement detection means for detecting whether the mobile node has moved from the network in which a mobile agent used by this mobile node exists to another IPv4 network or to an IPv6 network (network capable of executing communication by utilizing only the IPv6) or to an IPv4/v6 network, and movement status management means for managing the movement status so detected. 
     Since the mobile node according to this aspect of the invention automatically detects the kind of the network in which the mobile node itself exists at present and manages itself, the necessity for adding an IPv4 header to the message used for the IPv6 or the IPv6 packet can be judged appropriately. 
     According to still another aspect of the present invention, there is provided a mobile agent for assisting the movement of a mobile node executing communication by utilizing an IPv6, including IPv4 processing means for executing services in accordance with an IPv4, IPv6 processing means for executing services in accordance with the IPv6 and communication processing means for executing transmission/reception control of packets to and from networks, wherein the mobile agent further comprises mobile node management means for managing the IPv4 address of a mobile node usable in a foreign IPv4 network when receiving a message for use in the IPv6 for registering the movement, to which an IPv4 header transmitted from the mobile node to the IPv6 network to the mobile agent when the mobile agent moves to the IPv4 network is added, and movement assistance processing means for adding an IPv4 header, in which the IPv4 address of the mobile node usable in a foreign IPv4 network is set as a foreign address and the IPv4 address of the mobile agent is set as a home address, to the message used for the IPv6 to permit registration of the movement to the mobile node, and transmitting the message. 
     In the mobile agent according to the aspect of the invention described above, after the IPv4 header is added to the message used for the IPv6 and then the message is transmitted. Therefore, the message used for the IPv6 can be transmitted substantially to the mobile node that is moving to the IPv4 network. 
     According to still another aspect of the present invention, there is provided a mobile agent for assisting the movement of a mobile node executing communication by utilizing the IPv6, including IPv4 processing means for executing services in accordance with the IPv4, IPv6 processing means for executing services in accordance with the IPv6 and communication processing means for executing transmission/reception control of packets to and from networks, wherein the mobile agent further comprises transfer-to-other node processing means for deleting the IPv4 header when receiving an IPv4 encapsulated IPv6 packet transmitted by the mobile node, and transmitting again the IP packet so taken out to the network. 
     In the mobile agent according to the aspect of the invention described above, after only the IPv6 packet is taken out from the IPv4 encapsulated IPv6 packet, the IPv6 is again transmitted. Therefore, the IPv6 packet can be transmitted substantially from the mobile node, that is moving to the IPv4 network, to the node on the IPv6 network or on the IPv4/v6 network. 
     According to still another aspect of the present invention, there is provided a mobile agent for assisting the movement of a node executing communication by utilizing the IPv6, including IPv4 processing means for executing services in accordance with the IPv4, IPv6 processing means for executing services in accordance with the IPv6 and communication processing means for executing transmission/reception control of packets to and from networks, wherein the mobile agent further comprises transfer-to-other node processing means for generating an IPv4 encapsulated IPv6 packet by adding an IPv4 header, in which the IPv4 address of a foreign node usable in a foreign IPv4 network is set as a foreign IPv4 address and the IPv4 address of the mobile agent is set as a home IPv4 address, to the received IPv6 packet when receiving this IPv6 packet transmitted by other node to the mobile node that has moved to the IPv4 network, and for transmitting this IPv4 encapsulated IPv6 packet. 
     In the mobile agent according to the aspect of the invention described above, after the IPv4 header is added to the IPv6 packet, the IPv6 packet is transmitted. Therefore, the IPv6 packet can be transmitted substantially from the node on the IPv6 network or on the IPv4/v6 network to the mobile node that is moving to the IPv4 network. 
     According to still another aspect of the present invention, there is provided a network system in which an IPv4/v6 network and an IPv4 network are connected with each other by a connecting device or by the connection device and a third network, wherein the mobile agent according to the fourth, fifth or sixth aspect is provided on the IPv4/v6 network and the mobile node according to the first, second or third aspect is provided on the IPv4/v6 network or on the IPv4 network. 
     The network system according to the aspect described above can successively keep the network connection, which utilizes the IPv6 and has been already established before the movement of the IPv4/v6 node, when the IPv4/v6 node moves from the IPv4/v6 network to the IPv4 network, and can execute afresh communication by utilizing the IPv6. 
     According to still another aspect of the present invention, there is provided a method of controlling a mobile node by a mobile agent in a network system in which a first IP network capable of executing communication in accordance with first and second kinds of IPs and a second IP network capable of executing communication in accordance with only the first kind of IP, so that the mobile node capable of executing communication in accordance with the second kind of IP can communicate with other node belonging to the first IP network in accordance with the second kind of IP when the mobile node moves from the first IP network to the second IP network, which method comprises the steps of adding a first kind of IP header, in which the IP address of a second mobile agent belonging to the second IP network in accordance with the first kind of IP is set as a foreign address by the first mobile agent belonging to the first IP network and the IP address of the first mobile agent in accordance with the first kind of IP is set as a home address, to an IP packet transmitted in accordance with the second kind of IP from other node to the mobile node, and transmitting the IP packet to the second mobile agent; and deleting the first kind of IP header by the second mobile agent and transmitting the IP packet to the mobile node. 
     On the other hand, the IP packet may be transmitted to other node by adding the first kind of IP header, in which the IP address of the first mobile agent in accordance with the first kind of IP is set as a foreign address by the second mobile agent and the IP address of the second mobile agent in accordance with the first kind of IP is set as a home address, to the IP packet in accordance with the second kind of IP transmitted from the mobile node to other node, transmitting this IP address to the first mobile agent, deleting the first kind of IP header by the first mobile agent and then transmitting the IP packet to other node. 
     Alternatively, it is possible to employ a method comprising adding the first kind of IP header, in which the IP address of the first mobile agent in accordance with the first kind of IP is set as a foreign address by the second mobile agent and the IP address of the second mobile agent in accordance with the first kind of IP is set as a home address, to a movement registration request message in accordance with the second kind of IP that is received from the mobile node, transmitting this message to the first mobile agent, adding the first kind of IP header, in which the IP address of the second mobile agent in accordance with the first kind of IP is set as a foreign address by the first mobile agent and the IP address of the first mobile agent in accordance with the first kind of IP is set as a home address, to a message in accordance with the second kind of IP for permitting the movement, and transmitting this message to the second mobile agent. 
     The present invention provides also a network system for assisting the movement of the mobile node, having the features described above. 
     Furthermore, the present invention provide the first and second mobile agents for assisting the movement of the mobile node, having the features described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a structural view of a network system according to one embodiment of the present invention; 
     FIG. 2 is a structural view of a movement status management table used in an IPv4/v6 mobile node shown in FIG. 1; 
     FIG. 3 is a structural view of a mobile node management table used in an IPv6 mobile agent shown in FIG. 1; 
     FIG. 4 is a flowchart showing an IPv4/v6 movement processing in the IPv4/v6 mobile node shown in FIG. 1; 
     FIG. 5 is a flowchart showing a movement detection processing in the IPv4/v6 shown in FIG. 1; 
     FIG. 6 is a flowchart showing an IPv4 movement registration processing in the IPv4/v6 mobile node shown in FIG. 1; 
     FIG. 7 is a flowchart showing an IPv6 movement registration processing in the IPv4/v6 mobile node shown in FIG. 1; 
     FIG. 8 is a flowchart showing an IPv4-only movement registration processing in the IPv4/v6 mobile node shown in FIG. 1; 
     FIG. 9 is a flowchart showing an IPv6 packet transmission processing in the IPv4/v6 mobile node shown in FIG. 1; 
     FIG. 10 is a flowchart showing an IPv6 movement assistance processing in an IPv6 mobile agent shown in FIG. 1; 
     FIG. 11 is a flowchart showing a transfer-to-mobile node processing in the IPv6 mobile agent shown in FIG. 1; 
     FIG. 12 is a flowchart showing a transfer-to-other node processing in the IPv6 mobile agent shown in FIG. 1; 
     FIG. 13 is a structural view of an IPv6 movement registration request message; 
     FIG. 14 is a structural view of an IPv4 encapsulated IPv6 movement registration request message; 
     FIG. 15 is a structural view of an IPv4 encapsulated IPv6 packet; 
     FIG. 16 is a structural view of an IPv4 encapsulated IPv6 movement registration permission message; 
     FIG. 17 is a structural view of an IPv6 encapsulated IPv6 packet; 
     FIG. 18 is a structural view showing an example of a network to which the present invention is applied; 
     FIG. 19 is an explanatory view showing a structural example of a mobile node management table used in a home IPv6 mobile agent shown in FIG. 18; 
     FIG. 20 is an explanatory view showing a structural example of a mobile agent address table used in a foreign IPv6 mobile agent shown in FIG. 18; 
     FIG. 21 is an explanatory view showing a structural example of a movement assistance management table used in the foreign IPv6 mobile agent shown in FIG. 18; 
     FIG. 22 is an operation flowchart showing an example of the procedure of an IPv4 movement processing in an IPv4/v6 mobile node shown in FIG. 18; 
     FIG. 23 is an operation flowchart showing an example of the procedure of an IPv6 movement processing in the IPv4/v6 mobile node shown in FIG. 18; 
     FIG. 24 is an operation flowchart showing an example of the procedure of an IPv6 movement assistance processing in a home IPv6 mobile agent shown in FIG. 18; 
     FIG. 25 is an operation flowchart showing an example of the procedure of a foreign IPv6 mobile agent shown in FIG. 18; 
     FIG. 26 is an operation flowchart showing an example of the procedure of a transfer-to-foreign IPv6 mobile agent processing in the home IPv6 mobile agent shown in FIG. 18; 
     FIG. 27 is an operation flowchart showing an example of the procedure of a transfer-to-other node processing in the home IPv6 mobile agent shown in FIG. 18; 
     FIG. 28 is an operation flowchart showing an example of the procedure of a transfer-to-home IPv6 mobile agent processing in the foreign IPv6 mobile agent shown in FIG. 18; 
     FIG. 29 is an operation flowchart showing an example of the procedure of a transfer-to-mobile node processing in the foreign IPv6 mobile agent shown in FIG. 18; 
     FIG. 30 is an explanatory view showing a structural example of an IPv6 movement registration request message; 
     FIG. 31 is an explanatory view showing a structural example of a packet obtained by encapsulating an IPv6 encapsulated IPv6 packet by IPv4 encapsulation; 
     FIG. 32 is a structural view showing another example of a network to which the present invention is applied; 
     FIG. 33 is an explanatory view showing a structural example of a mobile node management table used in a home IPv4 mobile agent shown in FIG. 32; 
     FIG. 34 is an explanatory view showing a structural example of a mobile agent address table used in the foreign IPv4 mobile node shown in FIG. 32; 
     FIG. 35 is an explanatory view showing a structural example of a movement assistance management table used in the foreign IPv4 mobile agent shown in FIG. 32; 
     FIG. 36 is an operation flowchart showing an example of the procedure of an IPv4 movement assistance processing in a home IPv4 mobile agent shown in FIG. 32; 
     FIG. 37 is an operation flowchart showing an example of the procedure of the foreign IPv4 movement assistance processing in the foreign IPv4 mobile agent shown in FIG. 32; 
     FIG. 38 is an operation flowchart showing an example of the procedure of a transfer-to-foreign IPv4 mobile agent processing in a home IPv4 mobile agent shown in FIG. 32; 
     FIG. 39 is an operation flowchart showing an example of the procedure of a transfer-to-other node processing in the home IPv4 mobile agent shown in FIG. 32; 
     FIG. 40 is an operation flowchart showing an example of the procedure of a transfer-to-home IPv4 mobile agent in the foreign IPv4 mobile agent shown in FIG. 32; 
     FIG. 41 is an operation flowchart showing an example of the procedure of a transfer-to-mobile node processing in the foreign IPv4 mobile agent shown in FIG. 32; 
     FIG. 42 is an explanatory view showing a structural example of an IPv4 movement registration request message; 
     FIG. 43 is an explanatory view showing a structural example of a packet obtained by IPv6 encapsulation of an IPv4 movement registration permission message; 
     FIG. 44 is an explanatory view showing a structural example of a packet obtained by IPv6 encapsulation of an IPv4 movement registration request message; and 
     FIG. 45 is an explanatory view showing a structural example of a packet obtained by IPv6 encapsulation of an IPv4 packet. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be explained with reference to the accompanying drawings. 
     FIG. 1 is a structural view showing a network system according to one embodiment of the present invention. 
     This network system  1  includes a LAN (Local Area Network)-a  100  which makes use of both an IPv4 and an IPv6, a LAN-b  101  which makes use of only the IPv4 and a WAN (Wide Area Network)  102  which connects the LAN-a  100  and the LAN-b  101  by a public line or an exclusive line. 
     On the LAN-a  100  exist an IPv4 node  103 , an IPv6 node  104 , an IPv4 mobile agent-a  105  for assisting the movement of a node executing communication by utilizing the IPv4 by the procedure in accordance with a Mobile IPv4 between the networks, an IPv4/v6 mobile node  106  and an IPv6 mobile agent  107  for assisting the movement of the node which executes communication by utilizing the IPv4 and IPv6 and also executes communication by utilizing the IPv6 between the networks. The IPv6 mobile agent  107  functions also as a router and connects the LAN-a  100  and the WAN  102 . 
     An IPv4 mobile agent-b  108  and a router  109  exist on the LAN-b  101 . The router  109  connects the LAN-b  101  and the WAN  102 . 
     In this embodiment, the following IP addresses are allocated, respectively: 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 IPv4 address 
                 IPv6 address 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 LAN-a 100 
                 “10.0.0.0” 
                 “::11.0.0.0” 
               
               
                   
                 IPv4 node 103 
                 “10.0.0.10” 
               
               
                   
                 IPv6 node 104 
                   
                 “::11.0.0.30” 
               
               
                   
                 IPv4/v6 mobile node 106 
                 “10.0.0.1” 
                 “::11.0.0.1” 
               
               
                   
                 IPv4 mobile agent-a 105 
                 “10.0.0.11” 
               
               
                   
                 IPv6 mobile agent 107 
                 “10.0.0.20” 
                 “::11.0.0.20” 
               
               
                   
                 LAN-b 101 
                 “20.0.0.0” 
               
               
                   
                 IPv4 mobile agent-b 108 
                 “20.0.0.11” 
               
               
                   
                   
               
             
          
         
       
     
     The IPv4/v6 mobile node  106  includes an IPv4/v6 movement processing portion  114  for executing various processings when the node moves to another network, a movement detection processing portion  115  for executing a detection processing which detects the movement to another network, an IPv4 movement registration processing portion  116  for executing a movement notification processing which notifies the movement of the node to another IPv4 network or to an IPv4/v6 network, to the IPv4 mobile agent-a  105 , an IPv6 movement registration processing portion  117  for executing a movement notification processing which notifies the movement of the node to another IPv6 network or to the IPv4/v6 network, to the IPv6 mobile agent  107 , an IPv4-only movement registration processing portion  118  for executing a movement notification processing which notifies the movement of the node to another IPv4 network to the IPv6 mobile agent  107 , a movement status management table  119  for managing the movement status, an IPv4 processing portion  111  for executing a processing in accordance with the services offered by the IPv4, an IPv6 processing portion  112  for executing a processing in accordance with the services offered by the IPv6, an IPv6 packet transmission processing portion  113  for executing a transmission processing of the IPv6 packet, and a communication processing portion  110  for executing a transmission/reception control of the packet to and from the LAN. 
     Among the constituent elements of the IPv4/v6 mobile node  106  described above, the present invention disposes specifically the movement detection processing portion  114 , the IPv4-only movement registration processing portion  118 , the IPv6 packet transmission processing portion  113  and the movement status management table  119 . 
     The IPv6 mobile agent  107  includes an IPv6 movement assistance processing portion  121  which receives the movement report (a report representing the movement to the IPv6 network or to the IPv4/v6 network) from the IPv6 mobile node (not shown in the drawing) effecting communication by utilizing the IPv4/v6 mobile node  106  or IPv6 and moving between the networks, and assists the mobile node, a mobile node management table  126  for managing the movement status information of the mobile nodes, an IPv4 processing portion  122  for executing a processing in accordance with the services offered by the IPv4, a transfer processing portion  123  to another node, for transferring the packet which is transmitted by the IPv4/v6 mobile node  106  to the IPv6 node  104 , an IPv6 processing portion  124  for executing a processing in accordance with the services offered from the IPv6, a transfer processing portion  125  to a mobile node, for transferring the packet which is transmitted from the IPv6 node  104  to the IPv4/v6 mobile node  106 , and a communication processing portion  120  for executing transmission/reception control of the packet to the LAN. 
     Among the constituent elements of the IPv6 mobile agent  107  described above, it is the IPv6 movement assistance processing portion  121 , the transfer processing portion  123  to another node, the transfer processing portion  125  to a mobile node, and a mobile node management table  126  that constitute the characterizing part of the present invention. 
     FIG. 2 shows a structural example of the movement status management table  119 . 
     This movement status management table  119  has the following fields: 
     own IPv4 address  200 : 
     This is the IPv4 address of the IPv4/v6 mobile node  106  on the LAN-a  100  on which the IPv6 mobile agent  107  for assisting the movement of the IPv4/v6 mobile node  106  exists. 
     own IPv4 network address  201 : 
     This is the IPv4 network address of the LAN-a  100  on which the IPv6 mobile agent  107  for assisting the movement of the IPv4/v6 mobile node  106  exists. 
     own IPv6 address  202 : 
     This is the IPv6 address of the IPv4/v6 mobile node  106  on the LAN-a  100  on which the IPv6 mobile agent  107  for assisting the movement of the IPv4/v6 mobile node  106  exists. 
     own IPv6 network address  203 : 
     This is the IPv6 network address of the LAN-a  100  on which the IPv6 mobile agent  107  for assisting the movement of the IPv4/v6 mobile node  106  exists. 
     IPv4 mobile agent IPv4 address  204 : 
     This is the IPv4 address of the IPv4 mobile agent-a  105  on the LAN-a  100  on which the IPv4 mobile agent-a  105  for assisting the movement of the IPv4/v6 mobile node  106  exists. 
     IPv6 mobile agent IPv4 address  205 : 
     This is the IPv4 address of the IPv6 mobile agent  107  on the LAN-a  100  on which the IPv6 mobile agent  107  for assisting the movement of the IPv4/v6 mobile node  106  exists. 
     IPv6 mobile agent IPv6 address  206 : 
     This is the IPv6 address of the IPv6 mobile agent  107  on the LAN-a  100  on which the IPv6 mobile agent  107  for assisting the movement of the IPv4/v6 mobile node  106  exists. 
     post-movement IPv4 network address  207 : 
     This is the IPv4 network address of the network on which the IPv4/v6 mobile node  106  exists at the present moment. 
     pre-movement IPv4 network address  208 : 
     This is the IPv4 network address of the network before the IPv4/v6 mobile node  106  moves. 
     post-movement IPv6 network address  209 : 
     This is the IPv6 network address of the network in which the IPv4/v6 mobile node  106  exists at the present moment. When the network existing at present is the IPv4 network, “NULL” is set. 
     pre-movement IPv6 network address  210 : 
     This is the IPv6 network address of the network before the IPv4/v6 mobile node  106  moves. When the network before the movement is the IPv4 network, “NULL” is set. 
     Incidentally, the network address of the LAN-a  100  in which the IPv6 mobile agent  107  for assisting the movement of the IPv4/v6 mobile node  106  exists is set at the time of initialization to the field of each of the post-movement IPv4 network address  207 , the pre-movement IPv4 network address  208 , the post-movement IPv6 network address  209  and the pre-movement IPv6 network address  210 . 
     FIG. 3 shows a structural example of the mobile node management table  126 . 
     This mobile node management table  126  includes the following entries: 
     mobile node IPv6 address  30 : 
     This is the IPv6 address of the mobile node the movement of which is assisted by the IPv6 mobile agent  107 . 
     foreign IPv6 address  31 : 
     This is the IPv6 address on the network on which the mobile node exists at the present moment. When the network existing at present is the IPv4 network, “NULL” is set. 
     foreign IPv4 address  32 : 
     This is the IPv4 address on the network on which the mobile node exists at the present moment. When the network existing at present is the IPv6 network, “NULL” is set. 
     Incidentally, the entry of the mobile node does not exist in the mobile node management table  126  at the time of initialization. 
     FIG. 4 is a flowchart showing the IPv4/v6 movement processing  40  executed by the IPv4/v6 movement processing portion  114 . 
     Initialization of the movement status management table  119  is effected at Step  41 . 
     At the next Step  50 , the movement detection processing portion  115  is caused to repeatedly execute a movement detection processing  50 . 
     FIG. 5 is a flowchart showing the movement detection processing  50  executed by the movement detection processing portion  115 . 
     At Step  51 , the IPv4/v6 mobile node  106  transmits a message transmission request message for detecting the IPv4 movement and a message transmission request message for detecting the IPv6 movement, which request an IPv4 movement detection message and an IPv6 movement detection message for detecting the movement to another IPv4 network, the IPv6 network or the IPv4/v6 network, respectively. The IPv4 mobile agent and the IPv6 mobile agent that receive these message transmission request message for detecting the IPv4 movement and message transmission request message for detecting the IPv6 movement, respectively, transmit the IPv4 movement detection message and the IPv6 movement detection message, respectively. In addition, the IPv4 mobile agent and the IPv6 mobile agent periodically transmit the IPv4 movement detection message and the IPv6 movement detection message, respectively. 
     Next, a timer is set at Step  52 . 
     If the IPv4 movement detection message is received at Step  53 , the flow proceeds to Step  54  and when it is not, the flow proceeds to Step  55 . 
     At Step  54 , the network address of the network, to which the IPv mobile agent transmitting the received IPv4 movement detection message belongs is compared with the post-movement IPv4 network address  207  inside the movement status management table  119 . If they are the same network address, the flow proceeds to Step  55  and if they are different network addresses, the flow proceeds to Step  60 . 
     If the IPv6 movement detection message is received at Step  55 , the flow proceeds to Step  56  and if it is not, the flow proceeds to Step  57 . 
     At Step  56 , the network address of the network to which the IPv6 mobile agent transmitting the IPv6 movement detection message received belongs is compared with the post-movement IPv6 network address  209  inside the movement status management table  119 . If they are the same network address, the flow proceeds to Step  57  and if they are different network addresses, the flow proceeds to Step  70 . 
     At Step  57 , the flow returns to Step  53  if the time is not out, and proceeds to Step  58  if the time is out. 
     At Step  58 , whether or not the post-movement IPv4 network address  207  inside the movement status management table  119  and the pre-movement IPv4 network address  208  are different addresses and whether or not the post-movement IPv6 network address  209  and the pre-movement IPv6 network address are the same network address are judged, and if the result of this judgement proves Yes, the flow proceeds to Step  80  and if the result proves No, the processing is completed. 
     At Step  60 , the IPv4 movement registration processing portion  116  is caused to execute the IPv4 movement registration processing  60 . 
     At Step  70 , the IPv6 movement registration processing portion  117  is caused to execute the IPv6 movement registration processing  70 . 
     At Step  80 , the IPv4-only movement registration processing portion  118  is caused to execute the IPv4-only movement registration processing  80 . 
     The movement detection processing  50  described above will be explained more concretely. When the IPv4/v6 mobile node  106  exists on the LAN-a  100  at the present moment, it receives the IPv4 movement detection message and the IPv6 movement detection message transmitted by the IPv4 mobile agent-a  105  and by the IPv6 mobile agent  107 , respectively. In this instance, since the network address (=“10.0.0.0”) of the LAN-a  100  to which the IPv4 mobile agent-a  105  transmitting the IPv4 movement detection message belongs is the same as the post-movement IPv4 network address  207  (=10.0.0.0”) of the movement status table  119 , it is possible to know that the mobile node does not move to another IPv4 network or another IPv4/v6 network. Therefore, the flow proceeds from Step  54  to Step  55  but Step  60  (IPv4 movement registration processing) is not executed. Since the network address (=“:: 11.0.0.0”) of the network to which the IPv6 mobile agent  107  transmitting the IPv6 movement detection message belongs is the same as the post-movement IPv6 network address  209  (=“::11.0.0.0”) of the movement status table  119 , it is possible to know that the mobile node does not move to another IPv6 or another IPv4/v6 network. Therefore, the flow proceeds from Step  56  to Step  57  but Step  70  (IPv6 movement registration processing) is not executed. 
     Next, when the IPv4/v6 mobile node  106  has moved to the LAN-b  101  at the present moment, this mobile node  106  receives the IPv4 movement detection message transmitted by the IPv4 mobile agent-b  108 . Since the network address (=“20.0.0.0”) of the LAN-b  101  to which the IPv4 mobile agent-b  108  transmitting the IPv4 movement detection message belongs is different from the post-movement IPv4 network address  207  (=“10.0.0.0”) of the movement status table  119 , it is possible to know that the IPv4/v6 mobile node  106  has moved to another IPv4 network or another IPv4/v6 network. Therefore, the flow proceeds from Step  54  to Step  60 , where the IPv4 movement registration processing  60  is executed. As will be described later with reference to FIG. 6, the pre-movement IPv4 network address  208  of the movement status table  119  is updated to “10.0.0.0” and the post-movement IPv4 network address  207  is updated to “20.0.0.0”, by this IPv4 movement registration processing  60 . 
     On the other hand, because the IPv6 mobile agent does not exist in the LAN-b  101 , the IPv6 movement detection message is not received. In consequence, the flow proceeds from Step  55  to Step  57  and the processing of Steps  56  and  70  (IPv6 movement registration processing) is not executed. 
     Because the post-movement IPv4 network address  207  (=“20.0.0.0”) of the movement status table  119  is different from the pre-movement IPv4 network address  208  (=“10.0.0.0”) and because the post-movement IPv6 network address  209  (=“:: 11.0.0.0”) is the same as the pre-movement IPv6 network address  210  (=“:: 11.0.0.0”) after time-out, it is possible to know that the mobile node has moved to the IPv4 network. Therefore, the flow proceeds from Step  58  to Step  80  and the IPv4-only movement registration processing  80  is executed. 
     Incidentally, when the IPv4/v6 mobile node  106  moves to another IPv4/v6 network such as the LAN-a  100 , both of the IPv4 movement detection message and the IPv6 movement detection message are received. Therefore, both of the IPv4 movement registration processing  60  and the IPv6 movement registration processing  70  are executed. On the other hand, the post-movement IPv4 network address  207  of the movement status table  119  becomes inequal (≠) to the pre-movement IPv4 network address  208  and the post-movement IPv6 network address  209  becomes inequal (≠) to the pre-movement IPv6 network address  210 . Therefore, the flow does not proceed from Step  58  to Step  80  and the IPv4-only movement registration processing  80  is not executed. 
     FIG. 6 is a flowchart showing an example of the IPv4 movement registration processing executed by the IPv4 movement registration processing portion  116 . Incidentally, this IPv4 movement registration processing  60  is the processing which follows the processing procedure of the Mobile IPv4. 
     At Step  61 , the IPv4 network address  201  of the movement status management table  119  of its own is compared with the network address of the network to which the IPv4 mobile agent transmitting the IPv4 movement detection message belongs. When they are not the same network address, it is possible to know that the mobile node has moved to another network, and the flow proceeds to Step  62 . When they are the same network address, on the other hand, it is possible to know that the mobile node has returned to the LAN-a  100  in which the IPv6 mobile agent  107  assisting the movement of the IPv4/v6 mobile node  106  exists, and the flow then proceeds to Step  63 . 
     At Step  62 , the IPv4 address on the foreign network which the IPv4/v6 mobile node  106  can make use of is acquired. This IPv4 address can be acquired by utilizing a DHCP for executing automatic distribution of the addresses or by manual setting, for example. 
     At Step  63 , the IPv4 movement registration request message is transmitted to the IPv4 mobile agent registered to the IPv4 mobile node IPv4 address  204  of the movement status management table  119 . 
     At Step  64 , the movement registration permission message as the reply to the IPv4 movement registration request message is awaited from the IPv4 mobile agent, and after this IPv4 movement registration permission message is received, the flow proceeds to Step  65 . 
     At Step  65 , the post-movement IPv4 network address  207  of the movement status management table  119  is substituted for the pre-movement IPv4 network address  208  and then the network address of the network to which the IPv4 mobile agent transmitting the IPv4 movement detection message is substituted for the post-movement IPv4 network address  207 . 
     The IPv4 movement registration processing  60  described above will be explained more concretely. When the IPv4/v6 mobile node  106  moves from the LAN-a  100  to the LAN-b  101 , the flow proceeds from Step  61  to Step  62  and further to Step  63 , and transmits the IPv4 movement registration request message to the IPv4 mobile agent-a  105 . After the IPv4 movement registration permission is received from the IPv4 mobile agent-a  105 , the flow proceeds from Step  64  to Step  65 . Next, “10.0.0.0” is set to the pre-movement IPv4 network address  208  while “20.0.0.0” is set to the post-movement IPv4 network address  207 . 
     FIG. 7 is a flowchart showing an example of the IPv6 movement registration processing executed by the IPv6 movement registration processing portion  117 . Incidentally, this IPv6 movement registration processing  70  is the processing that follows the processing procedure of the Mobile IPv6. 
     At Step  71 , own IPv6 network address  203  of the movement status management table  119  is compared with the network address of the network to which the IPv6 mobile agent transmitting the IPv6 movement detection message belongs. When they are not the same network address, it is possible to know that the mobile node has moved to another network and the flow proceeds to Step  72 . When they are the same network address, on the other hand, it is possible to know that the mobile node has returned to the LAN-a  100  in which the IPv6 mobile agent  107  assisting the movement of the IPv4/v6 mobile node  106  exists, and the flow then proceeds to Step  73 . 
     At Step  72 , the IPv6 address on the foreign network which the IPv4/v6 mobile node  106  can make use of is acquired. Acquisition of this IPv6 address is made by utilizing the DHCP for executing automatic distribution of the addresses or by manual setting, for example. 
     At Step  73 , the IPv6 movement registration request message is transmitted to the IPv6 mobile agent registered to the IPv6 mobile agent IPv6 address  206  of the movement status management table  119 . This IPv6 movement registration request message contains its own IPv6 address  1301 , the foreign IPv6 address  1302  and the foreign IPv4 address  303  as shown in FIG.  13 . This IPv6 movement registration processing  70  sets the IPv6 address held by own IPv6 address  202  of the movement status management table  119  to its own IPv6 address  1301 , the foreign IPv6 address to the foreign IPv6 address  1302  and “NULL” to the foreign IPv4 address  1303 . 
     At Step  74 , the IPv6 movement registration permission message as the reply to the IPv6 movement registration request message is awaited from the IPv6 mobile agent, and after this permission message is received, the flow proceeds to Step  75 . 
     At Step  75 , the post-movement IPv6 network address  209  of the movement status management table  119  is substituted for the pre-movement IPv6 network address  210  and then the network address of the network to which the IPv6 mobile agent transmitting the IPv6 movement detection message belongs is substituted for the post-movement IPv6 network address  209 . 
     FIG. 8 is a flowchart showing an example of the IPv4-only movement registration processing executed by the IPv4-only movement registration processing portion  118 . 
     At Step  81 , the IPv4 encapsulated IPv6 movement registration request message is transmitted to the IPv6 mobile agent registered to the IPv6 mobile agent IPv6 address  206  of the movement status management table  119 . As shown in FIG. 14, this IPv4 encapsulated IPv6 movement registration request message contains an IPv4 header  1401  and an IPv6 movement registration request message  1300 . The IPv4 header  1401  contains in turn a foreign IPv4 address  1402  and a source IPv4 address  1403 . The address of the IPv6 mobile agent IPv4 address  205  of the movement status management table  119  is set to the foreign IPv4 address  1402  and the IPv4 address acquired in the foreign IPv4 network is set to the source IPv4 address  1403 . The IPv6 movement registration request message  1300  shown in FIG. 14 contains its own IPv6 address  1301 , the foreign IPv6 address  1302  and the foreign IPv4 address  1303  as shown in FIG.  13 . 
     The IPv4-only movement registration processing  80  sets the IPv6 address held by the IPv6 address  202  of the movement status management table  119  to its own IPv6 address  1301 , “NULL” to the foreign IPv6 address  1302  and the IPv4 address at the destination to the foreign IPv4 address  1303 . 
     At Step  82 , the IPv4 encapsulated IPv6 movement registration permission request message as the reply to the IPv4 encapsulated IPv6 movement registration request message is awaited from the IPv6 mobile agent, and after this IPv4 encapsulated IPv6 movement registration permission message is received, and the flow proceeds to Step  83 . Incidentally, the IPv4 processing portion  111  removes the IPv4 header from the IPv4 encapsulated IPv6 movement registration permission message (this procedure will be hereinafter called the “IPv4 decapsulation”) and delivers it to the IPv4-only movement registration processing portion  118 . This IPv4 decapsulation in the IPv4 processing portion  111  is one of the services offered by the existing IPv4. 
     At Step  83 , the post-movement IPv6 network address  209  of the movement status management table  119  is substituted for the pre-movement IPv6 network address  210  and then “NULL” is substituted for the post-movement IPv6 network address  209 . 
     The IPv4-only movement registration processing  80  described above will be explained more concretely. When the IPv4/v6 mobile node  106  has moved from the LAN-a  100  to the LAN-b  101 , the following IPv4 encapsulated IPv6 movement registration request message  1400  is generated at Step  81 . 
     IPv4 header: 
     foreign IPv4 address  1402 : “10.0.0.20” (IPv4 address of IPv6 mobile agent  107 ) 
     home IPv4 address  1403 : “20.0.0.1” (IPv4 address that the IPv4/v6 mobile node  106  uses afresh on the LAN-b  101 ) 
     IPv6 movement registration message  1300 : 
     own IPv6 address  1301 : “::11.0.0.1” 
     foreign IPv6 address  1302 : “NULL” 
     foreign IPv6 address  1303 : “20.0.0.1” 
     The IPv4 encapsulated IPv6 movement registration permission message  1400  is transmitted to the IPv6 mobile agent  107 . 
     Next, after the IPv4 encapsulated IPv6 movement registration permission message is received from the IPv6 mobile agent  107  at Step  82 , “::11.0.0.1” is set to the pre-movement IPv6 network address  210  at Step  83  and “NULL” is set to the post-movement IPv6 network address  209 . 
     FIG. 9 is a flowchart showing an example of the IPv6 packet transmission processing  90  executed by the IPv6 packet transmission processing portion  113  of the IPv6 processing portion  112  in the IPv4/v6 mobile node  106 . 
     At Step  91 , the IPv6 packet transmission request by the network application, etc., is awaited, and the flow proceeds to Step  92  if the transmission request is made. 
     At Step  92 , whether or not the IPv6 network address  209  after the movement of the movement status management table  119  is “NULL” is checked and if it is “NULL”, the flow proceeds to Step  93  and if it is not, the flow proceeds to Step  94 . 
     At Step  93 , since the destination is the IPv4 network, the IPv6 packet is encapsulated by IPv4 encapsulation and is transmitted. In other words, the IPv4 header  1401  is added to the IPv6 packet  1501  as shown in FIG. 15, the IPv6 mobile agent IPv4 address  205  of the movement status management table  119  is set to the foreign IPv4 address  1402  of that IPv4 header  1401 , the IPv4 address acquired by the foreign IPv4 network is set to the home IPv4 address, and the IPv4 encapsulated IPv6 packet  1500  is generated and transmitted. The flow then returns to Step  91  described above. 
     At Step  94 , since the destination is the IPv6 network or the IPv4/v6 network, the IPv6 is transmitted as such. The flow then returns to Step  91  described above. 
     The IPv6 packet transmission processing  90  will be explained more concretely. When the IPv4/v6 mobile node  106  moves from the LAN-a  100  to the LAN-b  101 , for example, the IPv4/v6 mobile node  106  receives the transmission request of the IPv6 packet  1501  by the network application at Step  91 . Then, “10.0.0.20” (IPv4 address of the IPv6 mobile agent  107 ) is set as the foreign IPv4 address to this IPv6 packet  1501  at Step  92  and furthermore, the IPv4 header  1401  to which “20.0.0.1” is set as the home IPv4 address  1403  is added. The IPv6 packet encapsulated by this IPv4 encapsulation is transmitted to the IPv6 mobile agent  107 . 
     FIG. 10 is a flowchart showing an example of the IPv6 movement assistance processing  1000  executed by the IPv6 movement assistance processing portion  121  of the IPv6 mobile agent  107 . 
     At Step  1001 , whether or not the message transmission request message for detecting the IPv6 movement is received from the IPv6 mobile node (not shown in the drawing) or the IPv4/v6 mobile node  106  is checked, and if it is, the flow proceeds to Step  1002  and if it is not, the flow proceeds to Step  1003 . 
     At Step  1002 , the IPv6 movement detection message is transmitted to the node which transmits the IPv6 movement detection message transmission request message described above. 
     At Step  1003 , whether or not the IPv6 movement registration request message  1300  is received is checked, and if it is, the flow proceeds to Step  1004  and if it is not, the flow returns to Step  1001 . 
     At Step  1004 , whether or not the movement registration request can be accepted is checked, and if it cannot be accepted, the flow proceeds to Step  1005  and if it can, the flow proceeds to Step  1006 . 
     At Step  1005 , the IPv6 movement registration rejection message is transmitted to the node that transmits the IPv6 movement registration request message  1300 . The flow then returns to Step  1001  described above. 
     At Step  1006 , own IPv6 address  1301  of the IPv6 movement registration request message  1300  is compared with the foreign IPv6 address  1302  and when they are the same address, the flow proceeds to Step  1007  and when they are different addresses, the flow proceeds to Step  1008 . 
     At Step  1007 , the information of the corresponding mobile node inside the mobile node management table  126  is deleted by judging that this mobile node returns to its own network. The flow then proceeds to Step  1011 . 
     At Step  1008 , the foreign IPv4 address  1303  inside the IPv6 movement registration request message  1300  is checked, and if “NULL” is set, the flow proceeds to Step  1009  and if it is not, the flow proceeds to Step  1010 . 
     At Step  1009 , the information of the mobile node is set to the mobile node management table  126  by judging that this mobile node moves to the IPv6 network or to the IPv4/v6 network. In other words, the value of the foreign IPv6 address  1302  inside the IPv6 movement registration request message  1300  so received is set to the foreign IPv6 address  31  inside the mobile node management table  126  and “NULL” is set to the foreign IPv4 address  32 . The flow then proceeds to Step  1011 . 
     At Step  1010 , the information of the corresponding mobile node is set to the mobile node management table  126  by judging that this mobile node has moved to the IPv4 network. In other words, “NULL” is set to the foreign IPv6 address  31  inside the mobile node management table  126  while the value of the foreign IPv4 address  1303  inside the IPv6 movement registration request message  1300  so received is set to the foreign IPv4 address  32 . The flow then proceeds to Step  1012 . 
     At Step  1011 , the IPv6 movement registration permission message is transmitted to the mobile node, and the flow returns to Step  1001  described above. 
     At Step  1012 , the IPv6 movement registration permission message encapsulated by IPv4 encapsulation is transmitted to the mobile node. In other words, as shown in FIG. 16, the IPv4 header  1401  is added to the IPv6 movement registration permission message  1601 , and the foreign IPv4 address  1303  inside the IPv6 movement registration request message  1300  is set to the foreign IPv4 address  1402  of the IPv4 header  1401 . Further, the IPv4 address of the IPv6 mobile agent  107  is set to the home IPv4 address  1403  and the IPv4 encapsulated IPv6 movement registration permission message is generated and transmitted. The flow then returns to Step  1001 . 
     Incidentally, when the IPv4/v6 mobile node  106  moves to the IPv4 network, the IPv4/v6 mobile node  106  transmits the IPv4 encapsulated IPv6 movement registration request message  1300  to the IPv6 mobile agent  107  as described already. When the IPv6 mobile agent  107  receives this IPv4 encapsulated IPv6 movement registration request message  1300 , IPv4 decapsulation of this message is executed at the IPv4 processing portion  122  and the IPv6 movement registration request message  1300  is taken out and delivered to the IPv6 movement assistance processing portion  121 . Since this processing is one of the services offered by the existing IPv4, any new function need not be added to the IPv4 processing portion  122 . 
     The IPv6 movement assistance processing  1000  described above will be explained more concretely. When the IPv4/v6 mobile node  106  has moved from the LAN-a  100  to the LAN-b  101 , the flow proceeds serially to Steps  1001 ,  1002 ,  1003  and  1004 , and since the foreign IPv6 address  1302  (=“NULL”) inside the IPv6 movement registration request message  1300  is different from own IPv6 address  1301  (=“::11.0.0.1”) at Step  1005 , the flow proceeds to Step  1008 . 
     At Step  1008 , since the foreign IPv4 address  1303  (=“20.0.0.1”) inside the IPv6 movement registration request message  1300  is not “NULL”, the flow proceeds to Step  1010 . At this Step  1010 , “::11.0.0.1” is registered to the mobile node IPv6 address  30  in the mobile node management table  126  as the information of the IPv4/v6 mobile node  106 , “20.0.0.1” is registered to the foreign IPv4 address  32 , and “NULL” is registered to the foreign IPv6 address  31 . At Step  1012 , the IPv4 header  1401  to which “20.0.0.1” is set as the foreign IPv4 address  1402  and “10.0.0.20” is set as the home IPv4 address  1403  is added to the IPv6 movement registration permission message  1601  and is transmitted to the IPv4/v6 mobile node  106 . 
     FIG. 11 is a flowchart showing an example of the transfer-to-mobile node processing  1100  which is executed by the transfer-to-mobile node processing portion  125  of the IPv6 processing portion  124  in the IPv6 mobile agent  107 . 
     At Step  1101 , reception of the IPv6 packet to the mobile node registered to the mobile node management table  126  among the IPv6 packets transmitted by the IPv6 node  104  and other IPv6 nodes (not shown in the drawing) is awaited, and after this packet is received, the flow proceeds to Step  1102 . 
     At Step  1102 , whether or not the foreign IPv6 address  31  of the corresponding mobile node inside the mobile node management table  126  is “NULL” is checked, and if it is “NULL”, the flow proceeds to Step  1103  and if it is not, the flow proceeds to Step  1104 . 
     At Step  1103 , the mobile node as the destination of the IPv6 packet is judged as moving to the IPv4 network, and the IPv6 packet is encapsulated by IPv4 encapsulation and is transmitted to the IPv4 network to which the mobile node as the destination of this packet is moving. The structure of the IPv4 encapsulated IPv6 packet at this time is shown in FIG.  15 . The foreign IPv4 address  32  of the corresponding mobile node inside the mobile node management table  126  is set to the foreign IPv4 address  1402  and the IPv4 address of the IPv6 mobile agent  107  is set to the home IPv4 address  1403 . The flow then returns to Step  1101 . 
     At Step  1104 , the mobile node as the destination of the IPv6 packet is judged as moving to the IPv6 network or to the IPv4/v6 network, and the IPv6 header is added afresh to the IPv6 packet (this processing will be hereinafter called “IPv6 encapsulation”) and is transmitted to the IPv6 network or to the IPv4/v6 network to which the mobile node is moving. In other words, as shown in FIG. 17, the IPv6 header  1701  is added to the IPv6 packet  1704 , the foreign IPv6 address  31  of the corresponding mobile node inside the mobile node management table  126  is set to the foreign IPv6 address  1702  of its IPv6 header  1701 , the IPv6 address of the IPv6 mobile agent  107  is set to the home IPv6 address  1703  and the IPv6 encapsulated IPv6 packet  1700  is generated and transmitted. The flow then returns to Step  1101 . Incidentally, the processing procedure for encapsulating the IPv6 packet by the IPv6 encapsulation is the procedure that follows the Mobile IPv6. 
     The transfer-to-mobile node processing  1100  described above will be explained more concretely. When the IPv4/v6 mobile node  106  has moved from the LAN-a  100  to the LAN-b  101 , “::11.0.0.1” is set as the information of the IPv4/v6 mobile node  106  to the mobile node IPv6 address  30  inside the mobile node management table  126  by the IPv6 movement assistance processing  1000  described already, “NULL” is set to the foreign IPv6 address  31  and “20.0.0.1” is set to the foreign IPv4 address  32 . Therefore, when the IPv6 mobile agent  107  receives the IPv6 packet addressed to the IPv4/v6 mobile node  106 , it adds the header IPv4 header  1401 , in which “20.0.0.1” is set to the foreign IPv4 address  1402  and “10.0.0.20” is set to the home IPv4 address  1403 , to this IPv6 packet and transfers it to the IPv4/v6 mobile node  106  of the LAN-b  101 . This IPv4 encapsulated IPv6 packet  1500  is received by the IPv4/v6 mobile node  106 , is IPv4-decapsulated by the IPv4 processing portion  111  and is processed as the ordinary IPv6 packet. 
     In this way, even when the IPv4/v6 mobile node moves from the LAN-a  100  as the IPv4/v6 network to the LAN-b  101  as the IPv4 network, this mobile node can receive the IPv6 packet transmitted by the IPv6 node  104  of the LAN-a  100 . 
     FIG. 12 is a flowchart showing an example of the transfer-to-other node processing  1200  executed by the transfer-to-other node processing portion  123  of the IPv4 processing portion  122  in the IPv6 mobile agent  107 . 
     At Step  1201 , the mobile agent awaits the reception of the IPv4 packet addressed to its own (IPv6 mobile agent  107 ) and when this packet is received, the flow proceeds to Step  1202 . 
     At Step  1202 , whether or not the IPv4 packet so received is the IPv6 packet encapsulated by IPv4 encapsulation is checked, and when it is the IPv4 encapsulated IPv6 packet, the flow proceeds to Step  1203  and when it is not, the flow proceeds to Step  1205 . 
     At Step  1203 , whether or not the home node of the IPv4 encapsulated IPv6 packet is the mobile node registered to the mobile node management table  126  is checked, and if it is registered, the flow proceeds to Step  1204  and if it is not, the flow proceeds to Step  1205 . 
     At Step  1204 , the IPv4 encapsulated IPv6 packet is decapsulated by IPv4 decapsulation and is transmitted to the network where the node as the destination exists. The flow then returns to Step  1201 . 
     At Step  1205 , the IPv4 packet so received is discarded. The flow then returns to Step  1201 . 
     The transfer-to-other node processing  1200  described above will be explained more concretely. Let&#39;s consider the case where the IPv4/v6 mobile node  106  transmits the IPv6 packet to the IPv6 node  104 . In this instance, the IPv6 packet is subjected to IPv4 encapsulation by the IPv6 packet transmission processing  90  by using the IPv4 header  1401  in which “10.0.0.20” is set as the foreign IPv4 address  1402  (addressed to the IPv6 mobile agent  107 ) and “20.0.0.1” is set as the home IPv4 address  1403 , and the IPv4 encapsulated IPv6 packet is transmitted to the IPv6 mobile agent  107 . Receiving this packet, the IPv6 mobile agent  107  removes the IPv4 header  1401  of the IPv4 encapsulated IPv6 packet at Step  1204  after passing through Steps  1201 ,  1202  and  1203 , and transmits the IPv6 packet  1501  to the LAN-a  100  in which the IPv6 node  104  as the address exists. This IPv6 packet is received as the ordinary IPv6 packet by the IPv6 node  104 . 
     As described above, even when the mobile node has moved from the LAN-a  100  as the IPv4/v6 network to the LAN-b  101  as the IPv4 network, the IPv4/v6 mobile node  106  can transmit the IPv6 packet to the IPv6 node  104  of the LAN-a  100 . 
     Incidentally, communication utilizing the IPv4 between the IPv4/v6 mobile node  106  and other nodes can be carried out by the movement assistance of the nodes in the IPv6 by the IPv4 mobile agent- 1   105  and the IPv4 mobile agent-b  108  supporting the Mobile IPv4 as the existing method. 
     When the IPv4/v6 mobile node  106  returns from the LAN-b  101  to the LAN-a  100 , the IPv4/v6 mobile node  106  detects the movement to the IPv6 or to the IPv4/v6 network by the movement detection processing described above. The mobile node is judged as returning to the LAN-a  100  by the IPv6 movement registration processing  70 , and transmits the IPv6 movement registration request message  1300  in which “::11.0.0.1” is set to its own IPv6 address, “::11.0.0.1” which is the same as its own IPv6 address  1301  to the foreign IPv6 address  1302  and “NULL” to the foreign IPv4 address  1303 , to the IPv6 mobile agent  107 . 
     Receiving the IPv6 movement registration request message  1300 , the IPv6 mobile agent  107  judges that the IPv4/v6 mobile node  106  returns to the LAN-a  100  because its own IPv6 address inside the IPv6 movement registration request message  1300  is the same as the foreign IPv6 address  1302 , and omits the information on the IPv4/v6 mobile node  106  inside the mobile node management table  126 . As a result, the IPv4/v6 mobile node  106  can make communication utilizing the ordinary IPv6. 
     Incidentally, the IPv4/v6 mobile node  106  reports its return to the LAN-a  100  to the IPv4 mobile agent-a  105 , too, by the IPv4 movement registration request message in accordance with the Mobile IPv4 processing procedure and for this reason, communication utilizing the ordinary IPv4 can be made, too. 
     The embodiment given above automatically detects the movement between the networks by utilizing the IPv4 movement detection message and the IPv6 movement detection message, but it is also possible to employ the construction in which the user of the mobile node reports by himself to the movement detection processing portion  116  so as to execute the IPv4 movement registration processing  60 , the IPv6 movement registration processing  70  or the IPv4-only movement registration processing  80 . 
     Next, another embodiment of the present invention will be explained with reference to the drawings. 
     First, the explanation will be given on the case where the IPv4/v6 mobile node moves from the IPv4/v6 network to the IPv4 network. 
     A structural example of the network system to which the present invention is applied and a structural example of the mobile agent will be explained with reference to FIG.  18 . As shown in the drawing, the network system according to this embodiment includes a LAN-a  1800 , a LAN-b  1801  and a WAN  1802  that connects the LAN-a  1800  and the LAN-b  1801  by a public line or an exclusive line. On the LAN-a  1800  exist an IPv4 node  1803  which executes communication by utilizing only the IPv4 as a protocol of a network layer as the third layer of an OSI reference model, an IPv6 node  1804  which executes communication by utilizing only the IPv6, an IPv4 mobile agent-a  1805  which assists the movement between the networks for the nodes executing communication by utilizing the IPv4 in accordance with the procedure of the Mobile IPv4, an IPv4/v6 mobile node  1806  which executes communication by utilizing both IPv4 and IPv6 and moves between the networks, and a home IPv6 mobile agent  1807  which assists the movement of a node when the node executing communication by utilizing the IPv6 modes to another network. 
     On the LAN-b  1801  exist an IPv4 mobile agent-b  1808  and a foreign IPv6 mobile agent  1809  which assists the movement of a node when the node executing communication by utilizing the IPv4 and the IPv6 and executing communication by utilizing IPv6 comes to the LAN-b  1801 . 
     Incidentally, the home IPv6 mobile agent  1807  functions also as a router handling both of the IPv4 packet and the IPv6 packet and connects the LAN-a  1800  and the WAN  1802 . The router  1810  handling only the IPv4 packet connects the LAN-b  1801  and the WAN  1802 . Therefore, whereas both of the IPv4 packet and the IPv6 packet can come out from the networks beyond the router from the LAN-a  1800 , only the IPv4 packet can come out from the LAN-b  1801 . Incidentally, transmission/reception itself of the IPv4 packet and the IPv6 packet can be made inside the LAN-a  1800  and the LAN-b  1801 . 
     In this embodiment, the IP addresses are listed below: 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 IPv4 address 
                 IPv6 address 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 IPv4 node 1803 
                 “10.0.0.10” 
                   
               
               
                   
                 IPv6 node 1804 
                   
                 “11::20” 
               
               
                   
                 IPv4/v6 mobile node 1806 
                 “10.0.0.30” 
                 “11::30” 
               
               
                   
                 IPv4 mobile agent-a 1805 
                 “10.0.0.11” 
               
               
                   
                 home IPv6 mobile agent 1807 
                 “10.0.0.1” 
                 “11::1” 
               
               
                   
                 IPv4 mobile agent-b 1808 
                 “20.0.0.11” 
               
               
                   
                 foreign IPv6 mobile agent 1809 
                 “20.0.0.1” 
                 “21::1” 
               
               
                   
                   
               
             
          
         
       
     
     The IPv4/v6 mobile node  1806  comprises an IPv4 movement processing portion  1813  which executes a processing in accordance with the Mobile IPv4 when the node moves to another IPv4 network or to an IPv4/v6 network, an IPv6 movement processing portion  1815  which executes a processing in accordance with the Mobile IPv6 when the mobile node moves to another IPv6 network or to an IPv4/v6 network, an IPv4 processing portion  1812  which executes a processing in accordance with the services offered by the IPv4, an IPv6 processing portion  1814  which executes a processing in accordance with the services offered by the IPv6 and a communication processing portion  1811  which executes a transmission/reception control, etc. of a packet to the LAN. 
     The home IPv6 mobile agent  1807  comprises an IPv6 movement assistance portion  1817  which assists the movement for the mobile node (not particularly shown in the drawing) executing communication by utilizing the IPv6 and moving between the networks or for an IPv6 mobile node  1806 , a mobile node management table  1822  which manages the information of the mobile node that has moved to another IPv6 network or to the IPv4/v6 network, an IPv6 processing portion  1818  which executes a processing in accordance with the services offered by the IPv4, a transfer-to-other node processing portion  1819  which executes a transfer processing of the IPv6 packet, which is transferred from the foreign IPv6 mobile agent  1809  and is transmitted by the IPv4/v6 mobile node  1806 , to the IPv6 node as the destination, an IPv6 processing portion  1820  which executes a processing in accordance with the services offered by the IPv6, a transfer-to-foreign IPv6 mobile agent processing portion  1821  which executes a transfer processing of the IPv6 packet, which is transmitted from another IPv6 node to the IPv4/v6 mobile node  1806 , to the foreign IPv6 mobile agent  1809  and a communication processing portion  1816  which executes a transmission/reception control, etc. of the packet to the LAN. 
     The foreign IPv6 mobile agent  1809  comprises a foreign IPv6 movement assistance portion  1823  which assists the movement of the IPv4/v6 mobile node  1806  when this node  1806  moves to the network (LAN-b  1801 ) to which the foreign IPv6 mobile agent  1809  belongs, a movement assistance management table  1828  which manages the information of this mobile node  1806 , a mobile agent address table  1830  which registers the address information of the home IPv6 mobile agent  1807 , an IPv4 processing portion  1824  which executes a processing in accordance with the services offered by the IPv4, a transfer-to-mobile node processing portion  1825  which executes a processing for transferring the packet, which is transferred from the home IPv6 mobile agent  1807  and is addressed to the IPv4/v6 mobile node  1806 , to the IPv4/v6 mobile node  1806 , an IPv6 processing portion  1826  which executes a processing in accordance with the services offered by the IPv6, a transfer-to-home IPv6 mobile agent processing portion  1812  which executes a processing for transferring the IPv6 packet, which is transmitted by the IPv4/v6 mobile node  1810  to another IPv6 node, to the home IPv6 mobile agent  1807 , and a communication processing portion  1829  which executes a transmission/reception control, etc. of the packet to the LAN. 
     Among the constituent elements of the home IPv6 mobile agent  1807  described above, it is the IPv6 movement assistance portion  1817 , the transfer-to-other node processing portion  1819 , the transfer-to-foreign IPv6 mobile agent processing portion  1821  and the mobile node management table  1822  that constitute the characterizing part of the present invention. Among the constituent elements of the foreign IPv6 mobile agent  1809 , it is the foreign IPv6 movement assistance portion  1823 , the transfer-to-mobile node processing portion  1825 , the transfer-to-home IPv6 mobile agent processing portion  1827 , the mobile agent address table  1830  and the mobile agent management table  1828  that constitute the characterizing part of the present invention. 
     FIG. 19 shows an example of the mobile node management table  1822 . As shown in this drawing, the mobile node management table  1822  includes a mobile node IPv6 address  1920  as the IPv6 address of the mobile node, the foreign IPv6 address  1921  representing the IPv6 address which the mobile node makes use of in the foreign IPv6 network or in the foreign IPv4/v6 network, and a foreign IPv6 mobile agent IPv4 address  1922  representing the IPv4 address of the foreign IPv6 mobile agent  109 . Here, when the mobile node moves to the IPv6 network or to the IPv4/v6 network, “NULL” is set to the foreign IPv6 mobile agent IPv4 address  1922  and when the mobile node moves to the IPv4 network, the IPv4 address of the foreign IPv6 mobile agent  1809  existing inside that network is set to the address  1922 . Incidentally, though the drawing shows the case where the entries for a plurality of mobile nodes exist, the entry of the mobile node does not exist in this table under the initial state. Further, the updating processing of this table will be described later. 
     FIG. 20 shows an example of the mobile agent address table  1830  described above. As shown in this drawing, the mobile agent address table  1830  includes the home IPv6 mobile agent IPv4 address  2030  and the home IPv6 mobile agent IPv6 address  2031  as the IPv4 address and the IPv6 address of all the home IPv6 mobile agents existing in the network system (though this embodiment represents only the home IPv6 mobile agent  1807  on LAN-a  1800 ). This table is set by a manager, for example. 
     FIG. 21 shows an example of the movement assistance management table  1828  described above. As shown in the drawing, the movement assistance management table  1828  includes a mobile node IPv6 address  2140  as the IPv6 address of the IPv4/v6 mobile node  1806 , a home IPv6 mobile agent IPv4 address  2141  as the IPv4 address of the home IPv6 mobile agent  1807  existing in the home network of the mobile node, and a registration flag  2142  representing whether the entry is “tentative registration” or “real registration”. Incidentally, though this drawing represents the case where the entries for a plurality of mobile nodes exist, the entry of the mobile node does not exist in this table under the initial state. The updating processing of this table will be described later. 
     In the construction described above, the processings of the IPv4/v6 mobile node  1806 , the home IPv6 mobile agent  1807  and the foreign IPv6 mobile agent  1809  when the IPv4/v6 mobile node  1806  moves from the LAN-a  1800  as the IPv4/v6 network to the LAN-b  1801  as the IPv4 network, and handling of each table described above, will be explained next in detail. 
     FIG. 22 is a flowchart showing an example of the processing of the IPv4 movement processing portion  1812  for detecting whether or not the IPv4/v6 mobile node  1806  has moved to another IPv4 network or to the IPv4/v6 network, and for executing various processings when the mobile node has moved. By the way, this IPv4 movement processing portion  1812  executes the processing in accordance with the processing procedure of the Mobile IPv4. 
     The IPv4 movement processing portion  1812  first transmits the message transmission request message for detecting the IPv4 movement as the message for requesting the transmission of the IPv4 movement detection message, which is in turn the message for detecting the movement of the mobile node to another IPv4 network or to the IPv4/v6 network (Step  2251 ). Incidentally, the IPv4 movement detection message is transmitted by the IPv4 mobile agent either periodically or when it receives the transmission request message of the IPv4 movement detection. Next, the IPv4 movement processing portion  1812  judges whether or not the IPv4 movement detection message is received (Step  2252 ). When the IPv4 movement detection message is received (Step  2252 YES), the IPv4 movement processing portion  1812  judges from this message whether or not the mobile node moves to another network (Step  2253 ). Incidentally, the network address information is set inside the IPv4 movement detection message, and the movement is detected by comparing this address information with the IPv4 address of the IPv4/v6 mobile node  1806  of its own. 
     When the movement of the mobile node to another network is found as a result of the judgement described above (Step  2253 YES), the IPv4 movement processing portion  1812  judges next whether or not the network as the visiting network is the home network of the IPv4/v6 mobile node  1806  (the LAN-a  1800  is the home network in this embodiment) (Step  2254 ). The IPv4 movement detection message is utilized at the time of this judgement, too. When it is not the home network as a result of this judgement, (Step  2254 NO), the IPv4 movement processing portion  1812  then acquires the foreign IPv4 address that is used by the IPv4 mobile node-a  1805  when it transfers the IPv4 packet bound to the IPv4/v6 mobile node  1806  to the mobile node that is moving to another network (Step  2255 ). The IPv4/v6 mobile node  1806  acquires this foreign IPv4 address from the addresses offered by the IPv4 mobile agent-b  1808  or by utilizing the DHCP that automatically distributes the addresses, or by manual setting. 
     To report and register the movement to the IPv4 mobile agent-a  1805 , the IPv4 movement processing portion  1812  transmits the IPv4 movement registration message (Step  2256 ). Thereafter, the IPv4 movement processing portion  1812  waits for the IPv4 movement registration permission message as the reply of the IPv4 movement registration request message from the IPv4 mobile agent-a  1805  (Step  2257 ) and after this message is received (Step  2257 YES), the flow returns again to the first step  2251 . The IPv4 movement processing portion  1812  repeats the processing described above. 
     FIG. 23 is a flowchart showing an example of the processing of the IPv6 movement processing portion  1815  for detecting whether or not the IPv4/v6 mobile terminal  1806  has moved to another IPv6 network or to the IPv4/v6 network and for executing various processings when this mobile node has moved. Incidentally, this IPv6 movement processing portion  1815  executes the processing in accordance with the procedure of the Mobile IPv6. 
     The IPv6 movement processing portion  1815  first transmits the message transmission request message for detecting the IPv6 movement, which is the message for requesting the transmission of the IPv6 movement detection message as the message for detecting the movement to another IPv6 network or to the IPv4/v6 network (Step  2361 ). Incidentally, this IPv6 movement detection message is transmitted by the IPv6 mobile agent either periodically or when it receives the message transmission request message for detecting the IPv6 movement. Next, the IPv6 movement processing portion  1815  judges whether or not the IPv6 movement detection message is received (Step  2362 ). When this IPv6 movement detection message is received (Step  2362 YES), the IPv6 movement processing portion  1815  judges from this message whether or not the mobile node has moved to another network (Step  2362 ). Incidentally, the network address information is set into the IPv6 movement detection message, and the movement detection is executed by comparing this address information with its own IPv6 address of the IPv4/v6 mobile terminal  1806 . 
     If the result of judgement represents that the mobile node has moved to another network (Step  2363 YES), the IPv6 movement processing portion  1815  judges next whether or not the visiting network is the home network (the LAN-a  1800  is the home network in this embodiment) (Step  2364 ). The IPv6 movement detection message is utilized for this judgement, too. When the destination of the movement is not the home network as a result of the judgement described above (Step  2364 NO), the IPv6 movement processing portion  1815  then acquires the IPv6 address that can be used in the visiting network. Acquisition of this IPv6 address is made by utilizing the DHCP which automatically distributes the address, by the address automatic generation function as one of the functions offered by the IPv6, or by manual setting. In order to report and register the movement to the home IPv6 mobile agent  1807 , the IPv6 movement processing portion  1815  transmits the IPv6 movement registration request message (Step  2366 ). 
     FIG. 30 shows the data structure of the IPv6 movement registration request message transmitted by the IPv4/v6 mobile node  1806 . As shown in the drawing, the IPv6 movement registration request message  3000  includes a IPv6 header  3001  and a IPv6 data  3004 . The IPv6 header  3001  includes a foreign IPv6 address  3002  and a home IPv6 address. The IPv6 address of the home IPv6 mobile agent  1807  is set to the home IPv6 address  3002 , and the IPv6 address which the IPv4/v6 mobile node  1806  acquires in the visiting network is set to the home IPv6 address  3003 . The IPv6 data  3004  includes the IPv6 address  3005  as the IPv6 address of the node itself transmitting this message and the foreign IPv6 address  3006  as the IPv6 address which the mobile node acquires afresh in the visiting network. When the IPv4/v6 mobile node  1806  returns to the LAN-a  1800  as the home network, the same address as its own IPv6 address  3005  is set to the foreign IPv6 address  3006 . 
     Thereafter, the IPv6 movement processing portion  1815  awaits until the IPv6 movement registration permission message as the reply of the IPv6 movement registration request message  3000  is received from the home IPv6 mobile agent  1807  (Step  2367 ) and after this message is received (Step  2367 YES), the flow returns again to the initial Step  2361 . Thereafter, the IPv6 movement processing portion  1815  repeats the processing described above. 
     FIG. 24 is a flowchart showing an example of the processing of the IPv6 movement assistance processing portion  1817  which executes the assistance processing for the movement of the IPv6 mobile node (not particularly shown in the drawing) or the IPv4/v6 mobile node  1806  between the networks. 
     The IPv6 movement assistance processing portion  1817  first judges whether or not the IPv6 movement detection message transmission message is received (Step  2401 ). When this message is found received as a result of this judgement (Step  2401 YES), the IPv6 movement assistance processing portion  1817  transmits the IPv6 movement detection message (Step  2402 ). The IPv6 movement assistance processing portion  1817  then judges whether or not the IPv6 movement registration request message  3000  is received (Step  2403 ). If the message is found received as a result of judgement (Step  2403 YES), the IPv6 movement assistance processing portion further judges whether or not the request for this movement registration is acceptable (Step  2404 ). If the request is found unacceptable as a result of judgement (Step  2404 NO), the IPv6 movement assistance processing portion  1817  transmits the IPv6 movement registration rejection message as the registration rejection reply message of the IPv6 movement registration request message  3000  to the mobile node. 
     If the request is acceptable (Step  2404 YES), the IPv6 movement assistance processing portion  1817  then compares its own IPv6 address  3005  inside the message with the foreign IPv6 address (Step  2406 ). If they are found the same as a result of this comparison (Step  2406 YES), the IPv6 movement assistance processing portion  1817  judges that the mobile node has returned to the home network, and deletes the corresponding information of the mobile node inside the mobile node management table  1812  (Step  2407 ). Then, the IPv6 movement assistance processing portion  1817  transmits the IPv6 movement registration permission message as the registration permission reply message of the IPv6 movement registration request message  3000  to the mobile node (Step  2411 ). 
     When the Ipv6 address  3005  and the foreign IPv6 address  3006  are found as the different addresses as a result of comparison (Step  2406 NO), the IPv6 movement assistance processing portion  1817  further judges whether or not the IPv6 movement registration request message  300  so received is encapsulated by IPv4 encapsulation and transferred from the foreign IPv6 mobile agent  1809  (Step  2008 ). Incidentally, IPv4 encapsulation of the IPv6 movement registration request message  3000  by the foreign IPv6 mobile agent  1809  is effected by the later-appearing foreign IPv6 movement assistance processing portion  1823  inside the foreign IPv6 mobile agent  1809 . When the home IPv6 mobile agent  1807  receives this IPv4 encapsulated IPv6 movement registration request message  3000 , its own IPv4 processing portion  1818  executes IPv4 decapsulation and delivers the decapsulated message to the IPv6 movement assistance processing portion  1817 . This IPv4 decapsulation by the IPv4 processing portion  1818  is one of the services offered by the existing IPv4. 
     When the message is not judged as being transferred as a result of the judgement as to IPv4 decapsulation and transfer (Step  2408 NO), the IPv6 movement assistance processing portion  1817  judges that the mobile node has moved to another IPv6 network or to the IPv4/v6 network and sets the information of this mobile node to the mobile node management table  1822 . At this time, the value of the foreign IPv6 address  3006  inside the IPv6 movement registration request message  3000 , which is received, is set to the foreign IPv6 address  1921  inside the mobile node management table  1822  and “NULL” is set to the foreign IPv6 mobile agent IPv4 address  1922  (Step  2409 ). The IPv6 movement assistance processing portion  1817  then transmits the IPv6 movement registration permission message to the mobile node (Step  2411 ). 
     When the message is found as being IPv4 encapsulated and transferred as a result of the judgement described above (Step  2408 YES), the IPv6 movement assistance processing portion  1817  judges that the mobile node has moved to the IPv4 network and sets the information of this mobile node to the mobile node management table  1822 . At this time, the value of the foreign IPv6 address  3005  inside the IPv6 movement registration request message  3000 , which is transferred, is set to the foreign IPv6 address  1921  inside the mobile node management table  1822 , and the value of the home IPv4 address inside the IPv4 header, which is added to the IPv6 movement registration request message  3000  transferred, is set to the foreign IPv6 mobile agent IPv6 address  1922 . The IPv6 movement assistance processing portion  1817  then executes IPv4 encapsulation of the IPv6 movement registration permission message as the reply to the mobile node and transfers the message (Step  2412 ). 
     The structure of the IPv6 movement registration permission message which is IPv4 encapsulated at this time is the same as the structure  1600  shown in FIG.  16 . The foreign IPv6 mobile agent IPv4 address  1922  registered to the mobile node management table  1822  is set to the foreign IPv4 address  1402  inside the IPv4 header  1401 , and own IPv4 address of the home IPv6 mobile agent  1807  is set to the home IPv4 address  1403 . 
     The IPv6 movement assistance processing portion  1817  completes the processings as described above and repeats thereafter the processing described above. 
     FIG. 25 is a flowchart showing an example of the processing of the foreign IPv6 movement assistance processing portion  1823  which executes the movement assistance processing for the IPv4/v6 mobile node  1806  between the networks at the foreign IPv6 mobile agent  1809 . 
     The foreign IPv6 movement assistance processing portion  1823  first judges whether or not the message transmission request message for detecting the IPv6 movement is received (Step  2501 ). When this message is found received as a result of the judgement (Step  2501 YES), the foreign IPv6 movement assistance processing portion  1823  transmits the IPv6 movement detection message (Step  2502 ). Next, the foreign IPv6 movement assistance processing portion  1823  judges whether or not the IPv6 movement registration request message  3000  is received (Step  2503 ). If this message is found received as a result of the judgement (Step  2503 YES), the IPv6 movement assistance processing portion  1823  registers tentatively the information of this mobile node to the movement assistance management table  1828  (Step  1804 ). At this time, the value of own IPv6 address  3005  inside the IPv6 movement registration request message  3000  received is set to the mobile node IPv6 address  2140  of the movement assistance management table  1828 , and the value of the home IPv6 mobile agent IPv4 address  2030  corresponding to the foreign IPv6 address  3002  inside the IPv6 movement registration request message  3000  is set to the home IPv6 mobile agent IPv4 address  2141  by looking up the mobile agent address table  1830 . Further, “tentative registration” is set to the registration flag. The foreign IPv6 movement assistance processing portion  1823  executes IPv4 encapsulation of the IPv6 registration request message  3000  so received and transfers the encapsulated message to the home IPv6 mobile agent  1807  (Step  2505 ). 
     The structure of the IPv4 encapsulated IPv6 movement registration request message at this time is the same as the structure  1400  shown in FIG.  14 . The IPv4 address  2141  of the home IPv6 mobile agent  1807  registered to the movement assistance management table  1828  is set to the foreign IPv4 address  1402  in the IPv4 header  1401 , and own IPv4 address of the foreign IPv6 mobile agent  1809  is set to the home IPv4 address  1403 . 
     Incidentally, after movement, the IPv4/v6 mobile node  1806  always transmits once the packet to the foreign IPv6 mobile agent  1809  in accordance with the processing procedure of the Mobile IPv6. Therefore, the foreign IPv6 mobile agent  1809  can receive the IPv6 movement registration request message  3000  address to the home IPv6 mobile agent  1807 . 
     The foreign IPv6 movement assistance processing portion  1823  sets the timer (Step  806 ) and waits for the IPv6 movement registration permission message  1601  as the reply of the IPv6 movement registration request message  3000  for a predetermined time (Steps  2507  and  2510 ). Incidentally, the IPv6 movement registration permission message  1601  is encapsulated by IPv4 encapsulation and is transferred by the home IPv6 mobile agent  1807  as described above. 
     When the IPv6 movement registration permission message  1601  is received within the predetermined time (Step  2507 YES), the foreign IPv6 movement assistance processing portion  1823  updates the registration flag  2142  corresponding to the mobile node, which is previously registered tentatively to the movement assistance management table  1828 , to “real registration” assistance management table  1828 , to “real registration” (Step  2508 ). Further, the home foreign IPv6 movement assistance processing portion  1823  executes IPv4 decapsulation of the IPv6 movement registration permission message  1601  received and transfers this message to the IPv4/v6 mobile node  1806  (Step  2509 ). When the IPv6 movement registration permission message  1601  is not received within the predetermined time (Step  2510 YES), the foreign IPv6 movement assistance processing portion  1823  deletes the information of this mobile node from the movement assistance management table  1828  (Step  2511 ). The foreign IPv6 movement assistance processing portion  1823  completes the processings as described above and thereafter executes them repeatedly. 
     FIG. 26 is a flowchart showing an example of the processing of the transfer-to-foreign IPv6 mobile agent processing portion  1821  which transfers the IPv6 packet, which other IPv6 node transmits to the IPv6 mobile node or to the IPv4/v6 mobile node  1806 , to the foreign IPv6 mobile agent  1809  existing in the network to which the mobile node moves, at the home IPv6 mobile agent  1807 . 
     The transfer-to-foreign IPv6 mobile agent processing portion  1821  first judges whether or not the IPv6 packet, which is registered to the mobile node management table  1822  and is addressed to the mobile node, among the IPv6 packets transmitted by the IPv6 node  1804  or other IPv6 nodes (not shown particularly in the drawing) is received (Step  2601 ). If this packet is found received as a result of the judgement, the transfer-to-IPv6 mobile agent processing portion  1821  executes afresh IPv6 encapsulation of this packet (Step  2602 ). 
     The structure of the IPv6 packet encapsulated by IPv6 encapsulation at this time is the same as the structure  1700  shown in FIG.  17 . The corresponding foreign IPv6 address  1921  inside the movement assistance management table  1822  is set to the foreign IPv6 address  1702  inside the IPv6 header  1701  and the IPv6 address of the home IPv6 mobile agent  1807  of its own is set to the home IPv6 address  1703 . 
     The transfer-to-foreign IPv6 mobile agent processing portion  1821  judges next whether or not the foreign IPv6 mobile agent IPv4 address  1922  of the corresponding mobile node inside the mobile node management table  1822  is “NULL” (Step  2603 ). If the foreign IPv6 mobile agent IPv4 address  1922  is found “NULL” as a result of the judgement (Step  2603 NO), the transfer-to-foreign IPv6 mobile agent processing portion  1821  judges that the mobile node is moving to the IPv6 network or to the IPv4/v6 network and transmits as such the IPv6 encapsulated IPv6 packet  1700  (Step  2605 ). Incidentally, the processing procedures for executing IPv6 encapsulation of the IPv6 packet and transmitting the packet follow the procedures of the ordinary Mobile IPv6. 
     If the foreign IPv6 mobile agent IPv4 address  1922  is judged as being other than “NULL” as a result of the judgement (Step  2603 YES), the transfer-to-foreign IPv6 mobile agent processing portion  1821  judges that this mobile node is moving to the IPv4 network, executes further IPv4 encapsulation of the IPv6 packet which has been IPv6 encapsulated already, and transmits it to the foreign IPv6 mobile agent  1809  (Step  2604 ). 
     FIG. 31 shows the structure of the packet  3100  which is IPv4 encapsulated at this time. As shown in the drawing, this packet has the structure in which the IPv4 header  1401  is added afresh to the IPv6 encapsulated IPv6 packet  1700  shown in FIG.  17 . The value of the corresponding foreign IPv6 mobile agent IPv4 address  1922  inside the mobile node management table  1822  is set to the foreign IPv4 address  1402  inside the IPv4 header  1401  and the value of the IPv4 address of the home IPv6 mobile agent  1807  of its own is set to the home IPv4 address  1403 . 
     The transfer-to-foreign IPv6 mobile agent processing portion  1821  completes the processing and thereafter executes repeatedly the processing described above. 
     FIG. 27 is a flowchart showing an example of the processing executed by the transfer-to-other node processing portion  1819  when the IPv6 packet, which the IPv4/v6 mobile node  1806  transfers to other IPv6 node on the foreign IPv4 network, is IPv4 encapsulated and transferred from the foreign IPv6 mobile agent  1809 , to the foreign IPv6 node, in the home IPv6 mobile agent  1807 . 
     The transfer-to-other node processing portion  1819  first judges whether or not the IPv4 packet addressed to the home IPv6 mobile agent  1807  itself is received (Step  2701 ). If it is found received as a result of judgement (Step  2701 YES), the transfer-to-other node processing portion  1819  then judges whether or not the packet so received is encapsulated by IPv4 encapsulation and transferred by the foreign IPv6 mobile agent  1809  (Step  2702 ). Incidentally, the transfer of the IPv6 packet by the foreign IPv6 mobile agent  1809  is executed by the transfer-to-home IPv6 mobile agent processing portion  1827  inside the foreign IPv6 mobile agent  1809  as will be described later. If it is not found the transferred IPv6 packet as a result of judgement (Step  2702 NO), the transfer-to-other node processing portion  1819  discards this packet (Step  2705 ). If it is the transferred IPv6 packet (Step  2702 YES), the transfer-to-other node processing portion  1819  further judges whether or not the home node of this IPv6 packet is the mobile node registered to the mobile node management table  1822  (Step  2703 ). If it is not found registered as a result of this judgement (Step  2703 NO), the transfer-to-other node processing portion  1819  discards this packet (Step  2705 ). If it is found registered (Step  2703 YES), the transfer-to-other node processing portion  1819  decapsulates this packet by IPv4 decapsulation and transmits it to the foreign IPv6 node (Step  2704 ). 
     The transfer-to-other node processing portion completes the processing and thereafter executes repeatedly the processing described above. 
     FIG. 28 is a flowchart showing an example of the processing executed by the transfer-to-home IPv6 mobile agent processing portion  1827  for transferring the IPv6 packet, which is transmitted by the IPv4/v6 mobile node  1806  to other IPv6 node in the foreign IPv6 mobile agent  1809 , to the home IPv6 mobile agent  107 . 
     The transfer-to-home IPv6 mobile agent processing portion  1827  first judges whether or not the IPv6 packet transmitted from the IPv4/v6 mobile node  106  registered to the movement assistance management table  1828  is received (Step  2801 ). If the corresponding packet is found received as a result of this judgement, the transfer-to-home IPv6 mobile agent processing portion  1827  then judges whether or not the registration flag  2142  of the corresponding mobile node inside the mobile node management table  1828  is “real registration” (Step  2802 ). If it is found the “real registration” as a result of this judgement (Step  2802 YES), the transfer-to-home IPv6 mobile agent processing portion  1827  then encapsulates the IPv6 packet so received by IPv4 encapsulation and transmits it to the home IPv6 mobile agent  1807  (Step  2803 ). 
     The structure of the IPv6 packet which is IPv4 encapsulated at this time is the same as the structure  1500  shown in FIG.  15 . 
     The value of the corresponding home IPv6 mobile agent IPv4 address  2141  inside the movement assistance management table  1828  is set to the foreign IPv4 address  1402  inside the IPv4 header  1401 , while own IPv4 address of the foreign IPv6 mobile agent  1809  itself is set to the foreign IPv4 address  1403 . 
     If the registration flag  2142  is not found the “real registration” as a result of the judgement (Step  2802 NO), the transfer-to-home IPv6 mobile agent processing portion  1827  discards the packet (Step  2804 ). The transfer-to-home IPv6 mobile agent processing portion  1827  completes the processing and thereafter executes repeatedly the processing described above. 
     FIG. 29 is a flowchart showing an example of the processing of the transfer-to-mobile node processing portion  1825  which executes the processing for transferring the packet to IPv4/v6 mobile node  1806  when the IPv6 packet, which is transmitted by other IPv6 mobile node to the IPv4/v6 mobile node  1806  by the home IPv6 mobile agent  1807  in the foreign IPv6 mobile agent  1809 , is encapsulated by IPv6 encapsulation, is further encapsulated by IPv4 encapsulation and is transferred. 
     The transfer-to-mobile node processing portion  1825  first judges whether or not the IPv4 packet addressed to the foreign IPv6 mobile agent  1809  is received (Step  2901 ). If the packet is found received as a result of this judgement (Step  2901 YES), the transfer-to-mobile node processing portion  1825  then judges whether or not the packet so received is the one encapsulated by IPv4 encapsulation and transferred by the home IPv6 mobile agent  1807  (Step  2902 ). Incidentally, the transfer of the IPv6 packet by this home IPv6 mobile agent  1807  is executed by the foreign IPv6 mobile agent processing portion  1821  described above. If the packet is not found as the transferred IPv6 packet as a result of the judgement (Step  2902 NO), the transfer-to-mobile node processing portion  1825  discards this packet (Step  2905 ). If it is found as the transferred packet (Step  2902 YES), the transfer-to-mobile node processing portion  1825  further judges whether or not the foreign node of this IPv6 packet is the mobile node really registered to the movement assistance management table  1828  (Step  2903 ). The IPv6 address of the foreign node is the address of the foreign node contained in the IPv6 packet  1704 . If it not found really registered as a result of the judgement (Step  2903 NO), the transfer-to-mobile node processing portion  1825  discards this packet (Step  2905 ). If it is really registered (Step  2903 YES), the transfer-to-mobile node processing portion  1825  decapsulates this packet by IPv4 decapsulation and then transfers it to the IPv4/v6 mobile node  1806  (Step  2904 ). 
     The transfer-to-mobile node processing portion  1825  completes the processing and thereafter executes repeatedly the processing described above. 
     The flow of the processings from FIGS. 22 to  29  described above will be explained hereby with reference to the network system shown in FIG.  18 . When the IPv4/v6 mobile node  1806  exists on the LAN-a  1800  as the home network, the IPv4/v6 mobile node  1806  receives the IPv4 movement detection messages and the IPv6 movement detection message transmitted by the IPv4 mobile agent-a  1805  and the home IPv6 mobile agent  1807 , respectively. Therefore, it is not judged as moving. 
     When the IPv4/v6 mobile node  1806  has moved to the LAN-b  1801 , the IPv4/v6 mobile agent  1806  receives the messages from the IPv4 mobile agent-b  1808  and the foreign IPv6 mobile agent  1809 , respectively. Therefore, the mobile is judged as having moved to other network. The IPv4/v6 mobile node  1806  transmits the IPv4 movement registration request message and the IPv6 movement registration request message  3000  to the IPv4 mobile agent-a  1805  and to the home IPv6 mobile agent  1807 , respectively, by the IPv4 movement processing portion  1813  and the IPv6 movement processing portion  1815 . 
     To this IPv6 movement registration request message  3000  are set “11::1” (home IPv6 mobile agent  1807 ) as the foreign IPv6 address  3002 , “21::30” (assumed as the IPv6 address used afresh on LAN-b  1801  by the IPv4/v6 mobile node  1806  in this embodiment) as the home IPv6 address  3003 , “11::30” (IPv4/v6 mobile node  1806 ) as its own IPv6 address  3005 , and “21::30” as the foreign IPv6 address  3006 . 
     In this embodiment, the IPv6 packet cannot come out from the LAN-b  1801  beyond the router as described above, but can transmit and receive the IPv6 packet inside the LAN-b  1801 . Therefore, the IPv4/v6 mobile node  1806  can receive the IPv6 movement detection message transmitted by the foreign IPv6 mobile agent  1809 , and can also transmit the IPv6 movement registration request message  3000  to the LAN-b  1801 . 
     The IPv6 movement registration request message  3000  is once received by the foreign IPv6 mobile agent  1809 . The foreign IPv6 mobile agent  1809  adds the IPv4 header  1401 , in which “10.0.0.1” (home IPv6 mobile agent  1807 ) is set as the foreign IPv4 address  1402  and “20.0.0.1” (foreign IPv6 mobile agent  1809 ) is set as the home IPv4 address  1403 , to the message by its foreign IPv6 movement assistance processing portion  1823 , and transfers the message to the home IPv6 mobile agent  1807 . Thereafter, this message is received by the home IPv6 mobile agent  1807 . After receiving this message, the home IPv6 mobile agent  1807  adds the IPv4 header  1401 , in which “20.0.0.1” (foreign IPv6 mobile agent  1809 ) is set as the foreign IPv4 address  1402  and “10.0.0.1” (home IPv6 mobile agent  1807 ) is set as the foreign IPv4 address  1403 , to the IPv6 movement registration permission message  1601  by its IPv6 movement assistance processing portion  1817 , and transmits this message to the home IPv6 mobile agent  1809 . Receiving this message, the foreign IPv6 mobile agent  1809  decapsulates this message by IPv4 decapsulation by the foreign IPv6 movement assistance processing portion  1823  and transmits decapsulated message to the IPv4/v6 mobile node  1806 . 
     In consequence, registration of the movement of the IPv4/v6 mobile node  1806  to the home IPv6 mobile agent  1807  is completed. At this time are set “11::30” to the mobile node IPv6 address  20 , “21::30” to the foreign IPv6 address  1921 , and “20.0.0.1” to the foreign IPv6 mobile agent IPv6 address  2140  of the mobile node management table  1822 , as the information of the IPv4/v6 mobile node  1806 . Similarly, “11::30” is set to the mobile node IPv6 address  2140  and “10.0.0.1”, to the home IPv6 mobile agent IPv4 address  2141  of the movement assistance management table  1828 . 
     When the home IPv6 mobile agent  1807  receives the IPv6 packet transmitted by the IPv6 node  1804  to the IPv4/v6 mobile node  1806 , it adds the IPv6 header  1701 , in which “21::30” is set to the foreign IPv6 address  1702  and “11::1” is set to the home IPv6 address  1703 , to this IPv6 packet by its transfer-to-foreign mobile agent processing portion  1821 , and further adds the IPv4 header  1401 , in which “20.0.0.1” is set to the foreign IPv4 address  1402  and “10.0.0.1” is set to the home IPv4 address  1403 , and transfers the packet to the foreign IPv6 mobile agent  1809 . The packet  3100  is received by the home IPv6 mobile agent  1809 . This mobile agent  1809  decapsulates this packet by IPv4 decapsulation by its transfer-to-mobile node processing portion  1825  and transmits it to the IPv4/v6 mobile node  1806 . The IPv4/v6 mobile node  1806  receives and processes this packet as the IPv6 packet in accordance with the ordinary Mobile IPv6 procedure. 
     When the home IPv6 mobile agent  1809  receives the IPv6 packet transmitted by the IPv4/v6 mobile node  1806  to the IPv6 node  1804 , on the contrary, it adds the IPv4 header  1401 , in which “10.0.0.1” (home IPv6 mobile agent  1807 ) is set to the home IPv4 address  1402  and “20.0.0.1” (foreign IPv6 mobile agent  1809 ) is set to the home IPv4 address  1403 , to this packet by the transfer-to-home IPv6 mobile agent processing portion  1827  and transmits the packet to the home IPv6 mobile agent  1807 . This IPv4 encapsulated packet  1500  is received by the home IPv6 mobile agent  1807 . The home IPv6 mobile agent  1807  decapsulates this packet by IPv4 decapsulation by its transfer-to-other node processing portion  1819  and transmits the packet to the foreign IPv6 node  1804 . The foreign IPv6 node  1804  receives and processes this packet as the ordinary IPv6 packet. 
     In the present invention, even when the IPv4/v6 mobile node  1806  moves from the LAN-a  1800  as the IPv4/v6 network to the LAN-b  1801  as the IPv4 network, the IPv4/v6 mobile node  1806  can receive the IPv6 packet transmitted from the IPv4/v6 mobile node  1804  to the IPv4/v6 mobile node  1806  as described above. On the contrary, the existing IPv6 node  1804  can receive the IPv6 packet transmitted by the IPv4/v6 mobile node  1806  to the IPv6 node  1804 . 
     Further, communication making use of the IPv4 between other nodes and the IPv4/v6 mobile node  1806  can be made by means of the movement assistance by the IPv4 mobile agent-a  1805  supporting the Mobile IPv4 as the existing method and the movement assistance on the IPv4 by the IPv4 mobile agent-b  1808 . 
     Incidentally, when the IPv4/v6 mobile node  1806  returns from the LAN-b  1801  to the LAN-a  1800 , the IPv4/v6 mobile node  1806  detects this return to the home network by the IPv6 movement processing portion  1815  described above. Then, the IPv4/v6 mobile node  1806  transmits the IPv6 movement registration request message  3000  in which “11::30” is set to its own IPv6 address  3005  and “11::30” which is the same as its own IPv6 address  3005  is set to the home IPv6 address  3006 , to the home IPv6 mobile agent  1807 . Receiving this IPv6 movement registration request message  3000 , the home IPv6 mobile agent  1807  judges that the IPv4/v6 mobile node has returned to the LAN-a  1800  as the home network because its own IPv6 address  3005  inside this message is the same as the foreign IPv6 address  3006 , and then deletes the information about this mobile node inside the mobile node management table  1822 . In consequence, the IPv4/v6 mobile node  1806  can execute communication utilizing the ordinary IPv6. Similarly, since the IPv4/v6 mobile node  1806  reports its return to the LAN-a  1800  to the IPv4 mobile agent-a  1805  in accordance with the processing procedure of the Mobile IPv4 by the IPv4 movement registration request message. Communication utilizing the ordinary IPv4 can be made, too. 
     In the embodiment described above, the movement of the mobile node between the networks is detected by utilizing the IPv4 movement detection message and the IPv4 detection message, but it is also possible to employ the system construction in which the user of the mobile node indicates by himself to the IPv4 movement processing portion  1813  and to the IPv6 movement processing portion and reports the movement to the IPv4 mobile agent and to the IPv6 mobile agent. 
     Next, the explanation will be given on the case where the IPv4/v6 mobile node moves from the IPv4/v6 network to the IPv6 network. 
     A structural example of the network system to which the present invention is applied and a structural example of the mobile agent will be described with reference to FIG.  32 . 
     As shown in this drawing, the network system according to this embodiment includes a LAN-c  3200 , a LAN-d  3201  and a WAN  1902  connecting the LAN-c  3200  and the LAN-d  3201  by a public line or an exclusive line. On the LAN-c  3200  exist an IPv4 node  3203  executing communication by utilizing only the IPv4, an IPv6 node  3204  executing communication by utilizing only the IPv6, an IPv4/v6 mobile node  1806  executing communication by utilizing both IPv4 and IPv6 and moving between the networks, a home IPv4 mobile agent-c  3206  executing communication by utilizing both IPv4 and IPv6 and assisting the movement of the node, which executes communication by utilizing the IPv4, between the networks, and an IPv6 mobile agent-c  3207  assisting the movement of the node, which executes communication by utilizing the IPv6 in accordance with the Mobile IPv6 procedure, between the networks. On the LAN-d  3201  exist a foreign IPv4 mobile agent  3208  which executes communication by utilizing the IPv4 and IPv6 and assists the movement of the node executing communication by utilizing the IPv4 when this node moves to the LAN-d  3201 , and an IPv6 mobile agent-d  3209 . Here, the IPv4/v6 mobile node  1806  is the same as the one shown in FIG.  18 . 
     Incidentally, the IPv6 mobile agent-c  3207  functions also as a router handling both of the IPv4 packet and the IPv6 packet and connects the LAN-c  3200  and the WAN  3202 . The IPv6 mobile agent-d  3209  functions also as a router handling only the IPv6 packet and connects the LAN-d  3201  and the WAN  3202 . Therefore, both of the IPv4 packet and the IPv6 packet can go out to the external networks beyond the routers, whereas only the IPv6 packet can go out from the LAN-d  3201 . Incidentally, transmission/reception itself of the IPv4 packet and the IPv6 packet inside the LAN-c  3200  and the LAN-d  3201  is possible. 
     In this embodiment, the IP addresses are tabulated below. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 IPv4 address 
                 IPv6 address 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 IPv4 node 3203 
                 “10.0.0.10” 
                   
               
               
                   
                 IPv6 node 3204 
                   
                 “11::20” 
               
               
                   
                 IPv4/v6 mobile node 1806 
                 “10.0.0.30” 
                 “11::30” 
               
               
                   
                 home IPv4 mobile agent 3206 
                 “10.0.0.1” 
                 “11::1” 
               
               
                   
                 home IPv4 mobile agent 3208 
                 “20.0.0.1” 
                 “21::1” 
               
               
                   
                   
               
             
          
         
       
     
     The home mobile agent  3206  includes an IPv4 movement assistance portion  3216  which executes communication by utilizing the IPv4 and assists the movement of an IPv4 mobile node (not particularly shown in the drawing) moving between the networks or an IPv4/v6 mobile node  1806 , a mobile node management table  3217  which manages the information of the mobile node that has moved to another IPv4 network or to the IPv4/v6 network, an IPv4 processing portion  3218  which executes processing in accordance with the services offered by the IPv4, a transfer-to-foreign IPv4 mobile agent processing portion  3219  which executes a processing for transferring the IPv4 packet, which is transmitted by other IPv4 node to the IPv4/v6 mobile node  1806 , to a foreign IPv4 mobile agent  3208 , an IPv6 processing portion  3220  which executes processing in accordance with the services offered by the IPv6, a transfer-to-other node processing portion  3221  which executes a processing for transferring the IPv4 packet, which is transferred from the foreign IPv4 mobile agent  3208  and is transferred to the IPv4/v6 mobile node  1806 , to the foreign IPv4 node, and a communication processing portion  3215  which executes transmission/reception control, etc. of the packet to and from the LAN. 
     The foreign IPv4 mobile agent  3206  comprises a foreign IPv4 movement assistance processing portion  3223  which assists the movement of the IPv4/v6 mobile node  1806  when this node  1806  moves to the network (LAN-d  3201 ) to which the foreign IPv4 mobile agent  3208  belongs, a movement assistance management table  3229  which manages the information of the mobile node, a mobile agent address table  3228  which registers the address information of the home IPv4 mobile agent  3206 , an IPv4 processing portion  3224  which executes a processing in accordance with the services offered by the IPv4, a transfer-to-mobile agent processing portion  3225  which executes a processing for transferring the IPv4 packet, which is transmitted from the IPv4/v6 mobile node  1806  to other IPv4 node, to the home IPv4 mobile agent  3206 , an IPv6 processing portion  3226  which executes a processing in accordance with the services offered by the IPv6, a transfer-to-mobile node processing portion  3227  which executes a processing for transferring the packet, which is transferred from the home IPv4 mobile agent  3206  to the IPv4/v6 mobile node  1806 , to the IPv4/v6 mobile node  1806 , and a communication processing portion  3222  which executes transmission/reception control, etc. of the packet to the LAN. 
     Here, among the constituent elements of the home IPv4 mobile agent  3206  described above, it is the IPv4 movement assistance processing portion  3216 , the mobile node management table  3217 , the transfer-to-foreign IPv4 mobile agent processing portion  3219  and the transfer-to-other node processing portion  3221  that constitute a characterizing part of the present invention. Among the constituent elements of the foreign IPv4 mobile agent  3208 , the constituent elements according to the present invention are the foreign IPv4 movement assistance portion  3223 , the mobile agent address table  3228 , the movement assistance management table  3229 , the transfer-to-home IPv4 mobile agent processing portion  3225  and the transfer-to-mobile node processing portion  3227 . 
     FIG. 33 shows an example of the mobile node management table  3217  described above. As shown in the drawing, the mobile node management table  3217  includes a mobile node IPv4 address  3300  as the IPv4 address of the mobile node, a foreign IPv4 address  3301  representing the foreign IPv4 address when the home IPv4 mobile agent  3206  transfers the IPv4 packet address to the mobile node when this mobile node is moving to another IPv4 network or to the IPv4/v6 network, and a foreign IPv4 mobile agent IPv6 address  3302  representing the IPv6 address of the foreign IPv4 mobile agent. Here, “NULL” is set to the foreign IPv4 mobile agent IPv6 address  3302  when the mobile node is moving to the IPv4 network or to the IPv4/v6 network, and the IPv6 address of the foreign IPv4 mobile agent  3208  existing inside the IPv6 network is set when the mobile node is moving to this IPv6 network. Incidentally, though this drawing illustrates the case where entries for a plurality of moving nodes exist, the entry of the mobile node does not exist under the initial state. The updating processing of this table will be later described. 
     FIG. 34 shows an example of the mobile agent address table  3228  described above. As shown in this drawing, the mobile agent address table  3228  comprises the IPv6 addresses of all the home IPv4 mobile agents existing in the network system (though only the home IPv4 mobile agent  3206  on the LAN-c  3200  is shown in this embodiment), the home IPv4 mobile agent IPv6 address  3400  as the IPv4 address and the home IPv4 mobile agent IPv4 address  3401 . This table is set by a manager, etc. 
     FIG. 35 shows an example of the movement assistance management table  3229  described above. As shown in this drawing, the movement assistance management table  3229  includes a mobile node IPv4 address  3500  as the IPv4 address of the IPv4/v6 mobile node  1806 , a home IPv4 mobile agent IPv6 address  3501  as the IPv6 address of the home IPv4 mobile agent  3206  existing inside the home network of the mobile node, and a registration flag  3502  representing whether the entry is “tentative registration” or “real registration”. Though this drawing illustrates the case where entries for a plurality of mobile nodes exist, the entry for the mobile node does not exist in this table under the initial state. The updating processing of this table will be described later. 
     In the construction described above, the processing operations of the IPv4/v6 mobile node  1806 , the home IPv4 mobile agent  3206  and the foreign IPv4 mobile agent  3208 , and handling of each table described above, when the IPv4/v6 mobile node  1806  has moved from the LAN-c  3200  as the IPv4/v6 network to the LAN-d  3201  as the IPv6 network, will be explained in detail. 
     FIG. 36 is a flowchart showing an example of the processing of the IPv4 movement assistance processing portion  3216  for executing the assistance processing of the IPv4 mobile node (not particularly shown in the drawing) or the IPv4/v6 mobile node  1806 , between the networks. 
     The IPv4 movement assistance processing portion  3216  first judges whether or not the message transmission request message for detecting the IPv4 movement is received (Step  3601 ). When this message is found received as a result of this judgement (Step  3601 YES), the IPv4 movement assistance processing portion  3216  transmits the IPv4 movement detection message (Step  3602 ). Next, the IPv4 movement assistance processing portion  3216  judges whether or not the IPv4 movement registration request message is received (Step  3603 ). Here, FIG. 42 shows the structure of this IPv4 movement registration request message  4200 . As shown in the drawing, the IPv4 movement registration request message  4200  includes an IPv4 header  1401  and an IPv4 data  4201 . The IPv4 header  1401  includes a foreign IPv4 address  1402  and a home IPv4 address  1403 , and the IPv4 address of the home IPv4 mobile agent  3206  is set to the foreign IPv4 address  1402  while the IPv4 address of the IPv4/v6 mobile node  1806  is set to the home IPv4 address  1403 . The IPv4 data  4201  includes the IPv4 address  4202  as own IPv4 address of the node transmitting this message and the foreign IPv4 address  4203  as the foreign address when the IPv4 packet address to this mobile agent is transferred. The same address as the IPv4 address  4202  is set to the foreign IPv4 address  4203  when the IPv4/v6 mobile node  1806  returns to the LAN-c  3200  as the home network. Incidentally, this message is transmitted by the IPv4 movement processing portion  1813  inside the IPv4/v6 mobile node  1806  explained already with reference to FIG.  22 . 
     When the IPv4 movement registration request message  4200  is found received as a result of judgement (Step  3603 YES), the IPv4 movement assistance processing portion  3216  further judges whether or not this movement registration request is acceptable (Step  3604 ). When it found unacceptable as a result of this judgement (Step  3604 NO), the IPv4 movement assistance processing portion  3216  transmits an IPv4 movement registration rejection message as a rejection reply message to the IPv4 movement registration request message  4200  to the mobile node (Step  3605 ). If it is found acceptable (Step  3604 YES), the IPv4 movement assistance processing  3600  then compares its own address  4202  inside the message with the foreign IPv4 address  4203  (Step  3606 ). 
     If own IPv4 address  4202  and the foreign IPv4 address  4203  are found the same as a result of the judgement described above (Step  3606 YES), the IPv4 movement assistance processing portion  3216  judges that the mobile node has returned to the home network and detects the information of the corresponding mobile node inside the mobile node management table  3217  (Step  3607 ). The IPv4 movement assistance processing portion  3216  transmits the IPv4 movement registration permission message as the permission reply message of registration of the IPv4 movement registration request message  4200  to the mobile node (Step  3611 ). 
     If own IPv4 address  4202  and the foreign IPv4 address  4203  are found different as a result of the judgement (Step  3609 NO), the IPv4 movement assistance processing portion  3216  further judges whether or not the IPv4 movement registration request message  4200  received is the message which is encapsulated by IPv6 encapsulation and transmitted by the foreign IPv4 mobile agent  3208  (Step  3608 ). Incidentally, this IPv6 encapsulation of the IPv4 movement registration request message  4200  by the foreign IPv4 mobile agent  3208  is executed by the foreign IPv4 movement assistance processing portion  3223  inside the later-appearing IPv4 mobile agent  3208 . Receiving this IPv4 movement registration request message  4200  which is IPv6 encapsulated in this way, the home IPv4 mobile agent  3206  decapsulates the message by IPv6 decapsulation by its IPv6 processing portion  3220  and delivers the message to the IPv4 movement assistance processing portion  3216 . IPv6 decapsulation by this IPv6 processing portion is one of the services offered by the existing IPv6. 
     If the result of the judgement represents that the message is not IPv6 encapsulated and is not transferred (Step  3608 NO), the IPv4 movement assistance processing portion  3216  judges that the mobile node has moved to another IPv4 network or to the IPv4/v6 network and sets the information of this mobile node to the mobile node management table  3217  (Step  3609 ). At this time, the value of the foreign IPv4 address  4203  inside the received IPv4 movement registration request message  4200  is set to the foreign IPv4 address  3301  inside the mobile node management table  3217  and “NULL” is set to the foreign IPv4 mobile agent IPv6 address  3302 . Then, the IPv4 movement assistance processing portion  3216  transmits the IPv4 movement registration permission message to the mobile node (Step  3611 ). 
     If the message is found the one that is IPv6 encapsulated and is transferred as a result of the judgement (Step  3608 YES), the IPv4 movement assistance processing portion  3216  judges that the mobile node has moved to the IPv6 network and sets the information of this mobile node to the mobile node management table  3217  (Step  3610 ). At this time, the value of the foreign IPv4 address  4203  inside the transferred IPv4 movement registration request message  3300  is set to the foreign IPv4 address  3301  inside the mobile node management table  3217 , and the value of the home IPv6 address inside the IPv6 added to the transferred IPv4 movement registration request message  4200  is set to the foreign IPv4 mobile agent IPv6 address  3302 . The IPv4 movement assistance processing portion  3216  encapsulates and transmits the IPv4 movement registration permission message as the reply to the mobile node (Step  3612 ). 
     The data structure of the IPv6 encapsulated IPv4 movement registration permission message  4301  at this time is shown in FIG.  43 . As shown in the drawing, this message has the construction in which the IPv6 header  1701  is added to the IPv4 movement registration permission message  4301 . The foreign IPv4 mobile agent IPv6 address  3302  registered to the mobile node management table  3217  is set to the foreign IPv6 address  1702  inside the IPv6 header  1701  and own IPv6 address of the home IPv4 mobile agent  3206  itself is set to the home IPv6 address  3003 . 
     The IPv4 movement assistance processing portion  3216  completes the processing and thereafter repeats the processing described above. 
     FIG. 37 is a flowchart showing an example of the processing of the foreign IPv4 movement assistance processing portion  3223  for executing the movement assistance processing of the IPv4/v6 mobile node  1806  between the networks in the foreign IPv4 mobile agent  3208 . 
     The foreign IPv4 movement assistance processing portion  3223  first judges whether or not the message transmission request message for detecting the IPv4 movement is judged (Step  3701 ). If this message is found received as a result of this judgement (Step  3701 YES), the foreign IPv4 movement assistance processing portion  3223  transmits the IPv4 movement detection message (Step  3702 ). Next, the foreign IPv4 movement assistance processing portion  3223  judges whether or not the IPv4 movement registration request message  4200  is received (Step  3703 ). If this message is found received as a result of the judgement (Step  3703 YES), the foreign IPv4 movement assistance processing portion  3223  tentatively registers the information of this mobile node to the movement assistance management table  3229  (Step  3704 ). At this time, the value of own IPv4 address  4202  inside the received IPv4 movement registration request message  4200  is set to the foreign IPv4 address  3500  inside the mobile node management table  3229  and the value of the home IPv4 mobile agent IPv6 address  3400 , that corresponds to the foreign IPv4 address  1402  inside the IPv4 movement registration request, message  4200 , is set to the home IPv4 mobile agent IPv6 address  3501  by looking up the mobile agent address table  3228 . Further, “tentative registration” is set to the registration flag  3502 . The foreign IPv4 movement assistance processing portion  3223  encapsulates by IPv6 encapsulation the IPv4 movement registration request message  4200  so received, and transfers the message to the home IPv4 mobile agent  3206  (Step  3705 ). 
     The structure of the IPv6 encapsulated IPv4 movement registration request message  4200  at this time is shown in FIG.  44 . As shown in this drawing, the message  4400  has the construction in which the IPv6 header  1701  is added to the IPv4 movement registration permission message  4200  shown in FIG.  42 . The home IPv4 mobile agent IPv6 address  3501  registered to the movement assistance management table  3229  is set to the foreign IPv6 address  1702  inside the IPv6 header  1701 , and own IPv6 address of the foreign IPv4 mobile agent  3208  is set to the home IPv6 address  1703 . 
     Incidentally, the IPv4/v6 mobile node  1806  always transmits after its movement the packet to the foreign IPv4 mobile agent  3208  in accordance with the processing procedure of the Mobile IPv4. Therefore, the foreign IPv4 mobile agent  3208  can receive the IPv4 movement registration request message  4200 . 
     The foreign IPv4 movement assistance processing portion  3223  sets the timer (Step  3706 ) and waits for the IPv4 movement registration permission message  4301  as the reply to the IPv4 movement registration request message  4200  for a predetermined time (Steps  3707  and  3710 ). By the way, this IPv4 movement registration permission message  4301  is encapsulated to the IPv6 encapsulated message and is transmitted by the home IPv4 mobile agent  3206  as described above. 
     If the IPv4 movement registration permission message  4301  is received within the predetermined time (Step  3707 YES), the foreign IPv4 movement assistance processing portion  3223  updates the registration flag  3502  corresponding to the mobile node, which has been tentatively registered to the mobile agent management table  3229  previously, to “real registration” (Step  3708 ). Further, the foreign IPv4 movement assistance processing portion  3223  decapsulates by IPv6 decapsulation the IPv6 header  1701  added to the received IPv4 movement registration permission message  4301  and transfers the message to the IPv4/v6 mobile node  1806  (Step  3709 ). If the IPv4 movement registration permission message  4301  is not received within the predetermined time (Step  3701 YES), the foreign IPv4 movement assistance processing portion  3223  deletes the information of this mobile node from the movement assistance management table  3229  (Step  3711 ). 
     The foreign IPv4 movement assistance processing portion  3223  completes the processing and thereafter repeats the processing described above. 
     FIG. 38 is a flowchart showing an example of the processing of the transfer-to-foreign IPv4 mobile agent processing portion  3219  which executes the processing for transferring the IPv4 packet transmitted by other IPv4 node to the IPv4 mobile node (not particularly shown in the drawing) or to the IPv4/v6 mobile agent  1806  to the foreign IPv4 mobile agent  3208  existing in the foreign network of the mobile node, in the home IPv4 mobile agent  3206 . 
     The transfer-to-foreign IPv4 mobile agent processing portion  3219  first judges whether or not the IPv4 packet addressed to the mobile node registered to the mobile node management table  3217  among the IPv4 packets transmitted by the IPv4 node  1804  and other IPv4 nodes (not particularly shown in the drawing) is received (Step  3801 ). If the corresponding packet is found received as a result of this judgement (Step  3801 YES), the transfer-to-foreign IPv4 mobile agent processing portion  3219  then judges whether or not the foreign IPv4 mobile agent IPv6 address  3302  of the corresponding mobile node inside the mobile node management table  3217  is “NULL” (Step  3802 ). If the foreign IPv4 mobile agent IPv6 address  3302  is found “NULL” as a result of the judgement (Step  3802 NO), the transfer-to-foreign IPv4 mobile agent processing portion  3219  judges that the mobile node is moving to the IPv4 network or to the IPv4/v6 network, and encapsulates the IPv4 packet so received by IPv4 encapsulation and transmits the encapsulated packet (Step  3804 ). Incidentally, the processing procedure for effecting IPv4 encapsulation and transferring the packet follows the ordinary Mobile IPv4. 
     If the foreign IPv4 mobile agent IPv6 address  3302  is found to be other than “NULL” as a result of the judgement (Step  3802 YES), the transfer-to-foreign IPv4 mobile agent processing portion  3219  judges that the mobile node is moving to the IPv6 network, encapsulates the received IPv4 packet by IPv6 encapsulation and transmits the encapsulated packet to the foreign IPv4 mobile agent  3208  (Step  3803 ). 
     The structure of the IPv6 encapsulated IPv4 packet at this time is shown in FIG.  45 . This packet has the construction in which the IPv6 header  1701  is added afresh to the IPv4 packet  4501 . The value of the foreign IPv4 mobile agent IPv6 address  3302  inside the mobile node management table  3217  is set to the foreign IPv6 address  1702  inside the IPv6 header  1701 , and own IPv6 address of the home IPv4 mobile agent  3206  is set to the home IPv6 address  1703 . 
     The transfer-to-foreign IPv4 mobile agent processing portion  3219  completes the processing and thereafter executes repeatedly the processing described above. 
     FIG. 39 is a flowchart showing an example of the processing of the transfer-to-other node processing portion  3221  which executes the processing for transferring the packet to the IPv4 node when the IPv4 packet transmitted by the IPv4/v6 mobile node  1806  to other IPv4 node on the foreign IPv6 network is encapsulated by IPv6 encapsulation and transferred by the foreign IPv4 mobile agent  3208 , in the home IPv4 mobile agent  3206 . 
     The transfer-to-other node processing portion  3221  first judges whether or not the IPv6 packet address to the home IPv4 mobile agent  3208  itself is received (Step  3901 ). If the packet is found received as a result of this judgement (Step  3901 YES), the transfer-to-other node processing portion  3221  then judges whether or not the packet is the IPv4 packet that is encapsulated and transferred by the foreign IPv4 mobile agent  3208  (Step  3902 ). Incidentally, this transfer of the IPv4 packet by the foreign IPv4 mobile agent  3208  is executed by the transfer-to-IPv4 mobile agent processing portion  3225  inside the later-appearing foreign IPv4 mobile agent  3208 . If the packet is not found the transferred IPv4 packet as a result of the judgement (Step  3902 NO), the transfer-to-other node processing portion  3221  discards this packet (Step  3905 ). If it is found the transferred IPv4 packet (Step  3902 YES), the transfer-to-other node processing portion  3221  further judges whether or not the foreign node of this IPv4 packet is the mobile node registered to the mobile node management table  3217  (Step  3903 ). If it is not found registered as a result of the judgement (Step  3903 NO), the transfer-to-other node processing portion  3221  discards this packet (Step  3905 ). If it is found registered (Step  3903 YES), the transfer-to-other node processing portion  3221  decapsulates this packet by IPv6 decapsulation and transmits it to the foreign IPv4 node (Step  3904 ). 
     The transfer-to-other node processing portion  3221  completes the processing and thereafter repeats the processing described above. 
     FIG. 40 is a flowchart showing an example of the processing of the transfer-to-home IPv4 mobile agent processing portion  3225  which executes the processing for transferring the IPv4 packet, which the IPv4/v6 mobile node  1806  transmits to other IPv4 nodes, to the home IPv4 mobile agent  3206  in the foreign IPv4 mobile agent  3208 . 
     The transfer-to-home IPv4 mobile agent processing portion  3225  first judges whether or not the IPv4 packet, which is registered to the movement assistance management table  3229  and is transmitted by the IPv4/v6 mobile agent  1806 , is received (Step  4001 ). If the corresponding packet is found received as a result of this judgement (Step  4001 YES), the transfer-to-home IPv4 mobile agent processing portion  3225  then judges whether or not the registration flag  3502  of the corresponding mobile node inside the mobile node management table  3229  is “real registration” (Step  4002 ). If the registration flag is found the “real registration” as a result of the judgement (Step  4002 YES), the transfer-to-home IPv4 mobile agent processing portion  3225  encapsulates the received IPv4 packet by IPv6 encapsulation and transmits it to the home IPv4 mobile agent  3206  (Step  4003 ). 
     The IPv4 packet subjected to IPv6 encapsulation at this time has the same structure as the structure shown already in FIG.  45 . The value of the corresponding home IPv4 mobile agent IPv6 address  3501  inside the movement assistance management table  3229  is set to the foreign IPv6 address inside the IPv6 header  1701  and the IPv6 address of the foreign IPv4 mobile agent  3208  itself is set to the foreign IPv6 address  1703 . 
     If the registration flag  3502  is not found the “real registration” as a result of the judgement (Step  4002 NO), the transfer-to-home IPv4 mobile agent processing portion  3225  discards this packet (Step  4004 ). The transfer-to-home IPv4 mobile agent processing portion  3225  completes the processing and thereafter repeats the processing described above. 
     FIG. 41 is a flowchart showing an example of the processing of the transfer-to-other mobile node processing portion  3227  which executes the processing for transferring the packet to the IPv4/v6 mobile node  1806  when the IPv4 packet transmitted by other IPv4 node to the IPv4/v6 mobile node  1806  by the home IPv4 mobile agent  3206  is encapsulated by IPv6 encapsulation and is transferred, in the foreign IPv4 mobile agent  3208 . 
     The transfer-to-mobile node processing portion  3227  first judges whether or not the IPv6 packet addressed to the foreign IPv4 mobile agent  3208  itself is received (Step  4101 ). If it is found received as a result of this judgement (Step  4101 YES), the transfer-to-mobile node processing portion  3227  then judges whether or not the received packet is the IPv4 packet which is IPv6 encapsulated and transferred by the home IPv4 mobile agent  3206  (Step  4102 ). Incidentally, this transfer of the IPv4 packet by the home IPv4 mobile agent  3206  is executed by the home IPv4 movement assistance processing portion  3219  described above. If the packet is not the transferred IPv4 packet as a result of the judgement (Step  4102 NO), the transfer-to-mobile node processing portion  3227  discards this packet (Step  4105 ). If it is the transferred IPv4 packet (Step  4102 YES), the transfer-to-mobile node processing portion  3227  further judges whether or not the node of this IPv4 packet is the mobile node registered really to the movement assistance management table  3229  (Step  4103 ). If the node is not found registered really (Step  4103 NO) as a result of this judgement, the transfer-to-mobile node processing portion  3227  discards the packet (Step  4105 ). If it is found registered really (Step  4103 YES), the transfer-to-mobile node processing portion  3227  decapsulates this packet by IPv6 decapsulation and transfers the packet to the IPv4/v6 mobile agent  1806  (Step  4104 ). 
     The transfer-to-other node processing is completed and thereafter the processing described above is repeatedly executed. 
     The flow of the processings shown in FIG.  22  and in FIGS. 36 to  41  will be explained with reference to the network system shown in FIG.  32 . When the IPv4/v6 mobile node  1806  exists on the LAN-c  3200  as the home network, the IPv4/v6 mobile node  1806  is judged as not moving because it receives the IPv4 movement detection message and the IPv6 movement detection message transmitted by the home IPv4 mobile agent  3206  and the IPv6 mobile agent-c  3207 , respectively. 
     When the IPv4/v6 mobile node  1806  has moved to the LAN-d  3201 , the IPv4/v6 mobile node  1806  is judged as having moved to another network because it receives the IPv4 movement detection message and the IPv6 movement detection message transmitted by the foreign IPv4 mobile agent  3208  and the IPv6 mobile agent-d  3209 , respectively. Then, the IPv4/v6 mobile node transmits the IPv4 movement registration request message  4200  and the IPv6 movement registration request message  3000  by means of the IPv4 movement processing portion  1813  and the IPv6 movement processing portion  1815  to the home IPv4 mobile agent  3206  and to the IPv6 mobile agent-c  3207 , respectively. 
     To this IPv4 movement registration request message  4200  are set “10.0.0.1” (home IPv4 mobile agent  3206 ) as the foreign IPv4 address  1402 , “10.0.0.30” as its own IPv4 address  3202  and “20.0.0.30” (as the foreign IPv4 address which the IPv4/v6 mobile node  1806  acquires from the foreign IPv4 mobile agent  3208  in the foreign LAN-d  3201  in this embodiment), as the transfer IPv4 address. 
     In this embodiment, the IPv4 packet cannot come out from the LAN-d  3201  beyond the router to the external network as described above but can transmit/receive the IPv4 packet inside the LAN-d  3201 . Therefore, the IPv4/v6 mobile node  1806  can receive the IPv4 movement detection message transmitted by the foreign IPv4 mobile agent  3208  and can also transmit the IPv4 movement registration request message  4200  to the LAN-d  3201 . 
     This IPv4 movement registration request message  4200  is once received by the foreign IPv4 mobile agent  3208 . The foreign IPv4 mobile agent  3208  adds the IPv6 header  1701 , in which “11::1” (home IPv4 mobile agent  3206 ) is set as the foreign IPv6 address  1702  and “21::1” (foreign IPv4 mobile agent  3208 ) is set as the home IPv6 address  1703 , to this message  4200  by means of its foreign IPv4 movement assistance processing portion  3223 , and transfers the message to the home IPv4 mobile agent  3206 . Thereafter, this message is received by the home IPv4 mobile agent  3206 . After receiving this message, the home IPv4 mobile agent  3206  adds the IPv6 header  1701 , in which “21::1” (foreign IPv4 mobile agent  3206 ) is set as the foreign IPv6 address  1702 ) and “11::1” (home IPv4 mobile agent  3208 ) is set as the home IPv6 address  1703 , to the IPv4 movement registration permission message  4301  by means of its IPv4 movement assistance processing portion  3216 , and transfers the message to the foreign IPv4 mobile agent  3208 . Receiving this message, the foreign IPv4 mobile agent  3208  decapsulates the message by IPv6 decapsulation by its foreign IPv4 movement assistance processing portion  3223  and transmits the message to the IPv4/v6 mobile node  1806 . 
     In this way, registration of the movement of the IPv4/v6 mobile node  1806  to the home IPv4 mobile agent  3206  is completed. At this time, “10.0.0.30” is set as the information of the IPv4/v6 mobile node  1806  to the mobile node IPv4 address  3300  of the mobile node management table  3217 , “20.0.0.30” is set to the foreign IPv4 address  3301  and “21::1” is set to the foreign IPv4 mobile agent IPv6 address  3302 . Further, “10.0.0.30” is set to the mobile node IPv4 address  3500  of the movement assistance management table  3229  and “11::1” is set to the foreign IPv4 mobile agent IPv6 address  3501 . 
     Receiving the IPv4 packet transmitted from the IPv4 node  3203  to the IPv4/v6 mobile node  1806 , the home IPv4 mobile agent  3206  adds the header  1701 , in which “21::1” (foreign IPv4 mobile agent  3208 ) is set to the foreign IPv6 address  1702  and “11::1” (home IPv4 mobile agent  3206 ) is set to the home IPv6 address  1703 , to the IPv4 packet by means of the transfer-to-foreign IPv4 mobile agent processing portion  3219 , and transfers the packet to the foreign IPv4 mobile agent  3208 . The IPv6 encapsulated packet is received by the foreign IPv4 mobile agent  3208 . The foreign IPv4 mobile agent  3208  decapsulates this packet by IPv6 decapsulation by its transfer-to-node processing portion  3227  and transmits it to the IPv4/v6 mobile node  1806 . The IPv4/v6 mobile node  1806  receives and processes this packet as the IPv4 packet in accordance with the procedure of the ordinary Mobile IPv4. 
     When the IPv4/v6 mobile node  106  receives the IPv4 packet transmitted to the IPv4 node  3203 , on the contrary, the foreign IPv4 mobile agent  3208  adds the IPv6 header  1701 , in which “11::1” (home IPv4 mobile agent  3206 ) is set to the foreign IPv6 address  1702  and “21::1” (foreign IPv4 mobile agent  3208 ) is set to the home IPv6 address  1703 , to the packet by means of the transfer-to-home IPv4 mobile agent processing portion  3205  and transmits the packet to the home IPv4 mobile agent  3206 . The IPv6 encapsulated packet is received by the home IPv4 mobile agent  3206 . The home IPv4 mobile agent  3206  decapsulates this packet by IPv6 decapsulation by its transfer-to-other node processing portion  3221  and then transmits it to the foreign IPv4 node  3203 . The IPv4 node  3203  receives and processes this packet as the ordinary IPv4 packet. 
     According to the present invention described above, even when the IPv4/v6 mobile node  1806  moves from the LAN-c  3200  as the IPv4/v6 network to the LAN-d  3201  as the IPv6 network, the IPv4/v6 mobile node  1806  can receive the IPv4 packet transmitted by the IPv4 node  3203  to the IPv4/v6 mobile node  1806 . On the contrary, the existing IPv4 node  3203  can receive the IPv4 packet transmitted by the IPv4/v6 mobile node  1806  to the IPv4 node  3203 . 
     Communication by making use of the IPv6 between other node and the IPv4/v6 mobile node  1806  can be made by the assistance of movement by the IPv6 mobile agent-c  3207  supporting the IPv6 and by the assistance of movement of the node in the IPv6 by the IPv6 mobile agent-d  3209 . 
     Incidentally, when the IPv4/v6 mobile node  1806  returns from the LAN-d  3201  to the LAN-c  3200 , the IPv4/v6 mobile node  1806  detects its return to the home network by the IPv4 movement processing  1813  described already. Then, the IPv4/v6 mobile node  1806  transmits the IPv4 movement registration request message, in which “10.0.0.30” is set to its own address  4202  and “10.0.0.30” having the same address as its own IPv4 address  4202  to the foreign IPv4 address  4203 , to the home IPv4 mobile agent  3206 . Receiving this IPv4 movement registration request message  4200 , the home IPv4 mobile agent  3206  judges that the IPv4/v6 mobile node  1806  has returned to the LAN-c  3200  as the home network because its own IPv4 address  4202  in the message has the same address as that of the foreign IPv4 address  4203 , and then deletes the information of this mobile node in the mobile node management table  3217 . As a result, the IPv4/v6 mobile node  1806  can make communication by utilizing the ordinary IPv4. Similarly, the IPv4/v6 mobile node  1806  reports the return to the LAN-c  3200  by the IPv6 movement registration request message  3000  to the IPv6 mobile agent-c  3207 , too, in accordance with the processing procedure of the Mobile IPv6. Therefore, communication utilizing the ordinary IPv6 can be made, as well.