Abstract:
In a wireless network in which an addressing server allocates an IP address, there are provided an IP address acquisition method, a communication apparatus and a roaming method whereby a terminal can automatically acquire an IP address from the addressing server. An IP address acquisition method for acquiring an IP address for a terminal  14  from a addressing server  18  comprises the steps of: providing the terminal  14  with a temporary IP address in a case that a first communication apparatus  10  is not capable of connecting with a second communication apparatus  12  when the first communication apparatus  10  receives an IP address request message from the terminal  14 ; discarding the temporary IP address from the terminal  14  when it becomes possible to connect with the second communication apparatus  12  and a communication path is established; and acquiring an IP address for the terminal  14  from the addressing server  18  via the communication path.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to communication apparatuses connecting between a terminal and a network by wireless, and more particularly to a communication apparatus, an IP address acquisition method and a roaming method in which the terminal can acquire an IP address from an addressing server. 
     2. Description of the Related Art 
     In order to communicate by using IP (Internet Protocol) in a network such as a LAN (Local Area Network), it is necessary to acquire and set an IP address for a terminal. There are two methods to set the IP address for the terminal; One is a method in which the IP address is manually set. The other is a method in which the IP address is set by using DHCP (Dynamic Host Configuration Protocol). When the IP address is manually set, a network administrator needs to manage IP addresses for terminals therein so that an identical IP address cannot be simultaneously allocated to a plurality of terminals. In contrast, under the DHCP, an IP address is dynamically and automatically allocated to a terminal. In the DHCP, when a terminal gains access to a DHCP server, an IP address is allocated for the terminal. The DHCP is currently used in a large number of networks. 
     On the other hand, a wireless LAN has been widespread recently. As shown in  FIG. 1 , in a wireless LAN, information is communicated by wireless between a station  10  connected with a PC  14  and an access point  12  connected with the LAN. In  FIG. 1 , the station is connected with the PC  14  by a fixed line, and the access point  12  is connected with a network by a fixed line. Also, a DHCP server  18  is connected with the network. 
     In such a wireless LAN, IP addresses have to be allocated to not only the PC  14  but also the station  10  and the access point  12  that are serving to implement wireless communication. 
     A description will be given of a procedure to set an IP address for the PC  14  by using the DHCP server  18  under this configuration. At the beginning, there are prepared the same number of IP addresses as the number of access points. In this configuration, just one IP address should be prepared because there is only one access point  12 . Next, a temporary IP address is manually set to the PC  14 . As the temporary IP address, there is provided an IP address whose network address (to be mentioned later) is identical to that of the prepared address. After receiving the temporary IP address, the PC  14  is engaged to set the prepared IP address to the station  10  and the access point  12 . Since this configuration has just the access point  12  therein, the station  10  and the access point  12  may have an identical IP address. Thus, the identical IP address is allocated to the station  10  and the access point  12 . 
     The station  10  and the access point  12  to which the identical IP address is allocated are connected with the PC  14  and a network  16 , respectively. In this manner, the PC  14  gains a connection with the network  16  via the station  10  and the access point  12 . After that, an IP address setting of the PC  14  is modified into a setting such that the PC  14  acquires an IP address through DHCP, and then the PC  14  is rebooted to apply the modification. In the above-mentioned procedure, the PC  14  can acquire an IP address from the DHCP server  18  via the station  10  and the access point  12 . 
     As mentioned above, even when the DHCP is used to set an IP address automatically in a wireless LAN, it is necessary to perform complicated tasks for setting some apparatuses to implement a wireless LAN and further reboot a PC to apply a new setting. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a communication apparatus, an IP address acquisition method and a roaming method in which the above-mentioned problems are eliminated. 
     A more specific object of the present invention is to provide a communication apparatus, an IP address acquisition method and a roaming method in which a terminal can automatically acquire an IP address from an addressing server in a wireless network necessary for the IP address. 
     In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention an IP address acquisition method for acquiring an IP address for a terminal from a dynamic addressing server in a communication system having a first communication apparatus connected with the terminal and a second communication apparatus connected with the dynamic addressing server wherein the first communication apparatus communicates via the second communication apparatus connected by wireless with the first communication apparatus, the method comprising the steps of: providing a temporary IP address to the terminal in a case where it is impossible to connect the first communication apparatus to the second communication apparatus when the first communication apparatus receives an IP address request message from the terminal; discarding the temporary IP address from the terminal when it is possible to connect the first communication apparatus with the second communication apparatus and establish a communication path; and acquiring an IP address for the terminal from the dynamic addressing server via the communication path. 
     In the above-mentioned IP address acquisition method, the first communication apparatus may serve for the step of providing a temporary IP address to the terminal and the step of discarding the temporary IP address from the terminal, and the terminal may serve for the step of acquiring an IP address for the terminal. 
     According to the above-mentioned invention, in a state that there is no connection between the first communication apparatus and the second communication apparatus and the first communication apparatus cannot connect to the second communication apparatus, when receiving an IP address request message from the terminal, the first communication apparatus provides the terminal with a temporary IP address. After establishing a connection with the second communication apparatus, the first communication apparatus makes the terminal discard the temporary IP address and the terminal can acquire an IP address from the dynamic addressing server via the established communication path. 
     Additionally, there is provided according to another aspect of the present invention an IP address acquisition method for acquiring an IP address for a terminal from a dynamic addressing server in a communication system having a first communication apparatus connected with the terminal and a second communication apparatus connected with the dynamic addressing server wherein the first communication apparatus communicates via the second communication apparatus connected by wireless with the first communication apparatus, the method comprising the steps of: establishing a communication path between the first communication apparatus and the second communication apparatus in a case where it is possible to connect the first communication apparatus to the second communication apparatus when the first communication apparatus receives an IP address request message from the terminal; transferring the IP address request message to the dynamic addressing server; and acquiring an IP address for the terminal from the dynamic addressing server via the communication path. 
     In the above-mentioned IP address acquisition method, the first communication apparatus may serve for the step of establishing a communication path between the first communication apparatus and the second communication apparatus and the step of transferring the IP address request message to the dynamic addressing server, and the terminal may serve for the step of acquiring an IP address for the terminal. 
     According to the above-mentioned invention, in a state that there is no connection between the first communication apparatus and the second communication apparatus, when receiving an IP address request message from the terminal, the first communication apparatus attempts to connect to the second communication apparatus. After establishing a communication path, the first communication apparatus sends the IP address request message to the dynamic addressing server via the established communication path. As a result, the terminal can acquire an IP address from the dynamic addressing server. 
     Additionally, there is provided according to another aspect of the present invention a roaming method for conducting a roaming in a communication system having a first communication apparatus connected with a terminal and a plurality of second communication apparatuses connected with a dynamic addressing server wherein the first communication apparatus has communicated via one of the second communication apparatuses that maintains a connection by wireless with the first communication apparatus and conducts a roaming from the one of the second communication apparatuses to another one of the second communication apparatuses, the method comprising the steps of: requesting a reconnection with the another one of the second communication apparatuses and establishing a communication path when the first communication apparatus detects a disconnection between the first communication apparatus and the one of the second communication apparatuses; discarding an IP address from the terminal when a network address of the one of the second communication apparatuses is different from a network address of the another one of the second communication apparatuses; and acquiring an IP address from the dynamic addressing server via the communication path established by a reconnection request of the first communication apparatus and conducting a roaming from the one of the second communication apparatuses to the another one of the second communication apparatuses. 
     In the above-mentioned roaming method, the first communication apparatus may serve for the step of requesting a reconnection with the another one of the second communication apparatuses and establishing a communication path and the step of discarding an IP address from the terminal, and the terminal may serve for the step of acquiring an IP address from the dynamic addressing server. 
     According to the above-mentioned invention, even if the first communication apparatus has already acquired an IP address from the dynamic addressing server, when the first communication apparatus detects a disconnection with one of the second communication apparatuses, the first communication apparatus can request a reconnection with another one of the second communication apparatuses, establish a communication path and accomplish a roaming from the one of the second communication apparatuses to the another one of the second communication apparatuses. Also, even if the terminal has already acquired an IP address from the dynamic addressing server, the terminal can acquire an IP address corresponding to a network address to which the another one of the second communication apparatuses belongs by discarding the old IP address if necessary. As a result, the roaming is successfully accomplished. 
     Additionally, there is provided according to another aspect of the present invention a communication apparatus for relaying by wireless between a terminal and a dynamic addressing server, comprising: a temporary IP address providing part providing the terminal with a temporary IP address when an IP address request message is received from the terminal; and a temporary IP address throwing part discarding the temporary IP address from the terminal. 
     According to the above-mentioned invention, when it is impossible to establish a communication path with the dynamic addressing server, the temporary IP address part makes it possible to communicate between the communication apparatus and the terminal. Once the communication path between the dynamic addressing server and the terminal is established, the temporary IP throwing part makes the terminal discard an IP address and send an IP address request message again. As a result, the terminal can acquire an IP address from the dynamic addressing server. 
     Additionally, there is provided according to another aspect of the present invention a communication apparatus for relaying by wireless between a terminal and a dynamic addressing server, comprising: a maintaining part maintaining an IP address request message when the IP address request message is received from the terminal; and a transferring part transferring the IP address request message maintained. 
     According to the above-mentioned invention, when an IP address request message is received from the terminal, if it is possible to establish a connection path between the terminal and the dynamic addressing server, the IP address request message is maintained. After the communication path is established, the maintained IP address request message is transferred to the dynamic addressing server and an IP address is allocated to the terminal. 
     Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration of a wireless LAN; 
         FIG. 2  is a diagram illustrating a configuration of a wireless LAN according to an embodiment of the present invention; 
         FIG. 3  is a block diagram illustrating hardware of a communication apparatus; 
         FIG. 4  is a sequence diagram illustrating how a DHCP server allocates an IP address in a case that it is impossible that a first communication apparatus is connected with a second communication apparatus when the first communication apparatus receives an IP address request message from a terminal; 
         FIG. 5  is a flowchart illustrating a process in an STA mode; 
         FIG. 6  is a flowchart illustrating a process in an AP mode; 
         FIG. 7  is a sequence diagram illustrating how a DHCP server allocates an IP address in a case that it is possible that a first communication apparatus is connected with a second communication apparatus when the first communication apparatus receives an IP address request message from a terminal; 
         FIG. 8  is a diagram illustrating a configuration wherein a roaming is conducted; and 
         FIG. 9  is a flowchart illustrating a roaming process. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention will be described with reference to the accompanying drawings. 
       FIG. 2  shows an entire configuration according to the embodiment of the present invention. In  FIG. 2 , there are illustrated communication apparatuses  10  and  12 , a PC  14  connected with the communication apparatus  10 , a network  16  connected with the communication apparatus  12 , and a DHCP server  18  connected with the network  16 . The communication apparatus  10  is connected with the PC  14  in a cable such as a 10Base-T, and the communication apparatus  12  is also connected with the network  16  in a cable such as a 10Base-T. 
     A description will now be given of the communication apparatuses  10  and  12 . The communication apparatuses  10  and  12  are formed of a similar type of communication apparatus, and can switch into a plurality of modes to be mentioned later.  FIG. 3  shows a block diagram illustrating hardware of the communication apparatuses  10  and  12 . The communication apparatuses  10  and  12  shown in  FIG. 3  comprise a CPU for control  20 , a frame buffer memory  22 , an Ethernet MAC Controller  24  for connecting to a network, a MAC part  26  in a MAC layer serving for a media access control in a MAC sub-layer, a PHY part  28  for serving as an interface with a physical layer, and an antenna  30 . In this configuration, data from the Ethernet MAC Controller  24  is temporally stored in the frame buffer memory  22 . Then, the MAC part  26  performs a media access control in accordance with the CSMA/CA method (IEEE802.11) for the data, and the data is formed as a frame in accordance with the IEEE802.11 and is delivered in the PHY part  28 , which is eventually propagated through the antenna  30 . On the other hand, data from the antenna  30  is lifted in the PHY part  28  from the physical layer to the MAC sub-layer, and the MAC part  26  performs a media access control for the data. Then, the data is temporally stored in the frame buffer memory  22  and is sent through the Ethernet MAC Controller  24 . The CPU for control  20  is responsible to control such a data receiving and sending process. 
     A description will now be given of the above-mentioned operational mode. As the operational mode, there are prepared at least two modes: a station mode (Hereinafter, notated as an STA mode) and an access point mode (Hereinafter, notated as an AP mode). The STA mode is a mode as the communication apparatus  10  for performing a connecting process with the PC  14 . The AP mode is a mode as the communication apparatus  12  for performing a connecting process with the network  16  and serving as an access point between the communication apparatus  10  and the network  16 . 
     A description will now be given of some settings regarding the communication apparatuses  10  and  12 . Among primary setting items regarding the communication apparatuses  10  and  12 , there are a mode setting regarding the above operational modes, an SSID setting, a cipher setting, an authentication setting, and a roaming setting. The SSID is a kind of a name of an apparatus and is an ID represented as a sequence formed of 32 characters of an ASCII code. The cipher setting, which is involved in ciphering data exchanged between the STA mode and the AP mode, sets a ciphering mode (a bit length, a type of cipher, and a length of key). The authentication setting, which is involved in authentication between an apparatus in the STA mode and an apparatus in the AP mode, sets an authentication algorithm. These settings can be set by connecting with a computer such as a PC. 
     The description will now be given of a way in which the PC  14  acquires an IP address from the DHCP server  18  by using the communication apparatuses  10  and  12  provided under the above-mentioned configuration. The description will be given separately from two cases: One case is that it is impossible to connect with the communication apparatus  12  when the communication apparatus  10  receives an IP address request from the PC  14 . The other is that it is possible to connect with the communication apparatus  12  when the communication apparatus  10  receives an IP address request from the PC  14 . It should be noted that the communication apparatuses  10  and  12  are represented as an STA  10  and an AP  12  in the following description, respectively. 
     A description will now be given, with reference to  FIG. 4 , of the case that it is impossible to connect with the communication apparatus  12  when the communication apparatus  10  receives an IP address request from the PC  14 . At the beginning, at step ( 1 ), the AP  12  sends a message “DHCP DISCOVER” (search) to search a DHCP server  18 . At step ( 2 ), the DHCP server  18  sends a message “DHCP OFFER” (reply for search) back to the AP  12  to show that the DHCP server  18  works. At step ( 3 ), the AP  12  sends a message “DHCP REQUEST” (IP address request) to the DHCP server  18 . At step ( 4 ), the DHCP server sends a message “DHCP ACK” (reply for IP address request) back to the AP  12 , and an IP address (for example, 172.16.40.10) is allocated to the AP  12  by the DHCP server  18 . On the other hand, when the DHCP server  18  cancels out an IP address that has been allocated to a DHCP client, the DHCP server  18  sends a message “DHCP RELEASE” to invalidate the IP address allocated. In the above manner, since the AP  12  has a connection with the network  16 , the above-mentioned some settings regarding the AP  12  can be manipulated from another PC via the network  16 . 
     A description will now be given of a correspondence between the PC  14  and the STA  10  being in the STA mode. Suppose that the PC  14  has such a setting that the PC  14  acquires an IP address by using DHCP. Further suppose that an IP address (for example, 192.168.1.1) is allocated to the SAT  10  as a default and that the SAT  10  uses the IP address until the SAT  10  knows an IP address of the AP  12  by establishing a connection with the AP  12 . Currently, the STA  10  and the AP  12  are assumed to have no connection each other. 
     In the above situation, at step ( 5 ), the PC  14  sends a message “DHCP DISCOVER” to the STA  10 . At step ( 6 ), the STA  10  sends a message “DHCP OFFER” back to the PC  14 . At step ( 7 ), the PC  14  sends a message “DHCP REQUEST” (IP address request) to the STA  10 . At step ( 8 ), the STA  10  sends a message “DHCP ACK” (reply for IV address request) back to the PC  14  and a temporary IP address (for example, 192.168.1.2) is allocated to the PC  14  by the STA  10 . In the above manner, since the STA  10  has a connection with the PC  14 , the above-mentioned some settings regarding the STA  10  can be manipulated from the PC  14 . Now, since the STA  10  and the PC  14  are connected each other, if the STA  10  and the AP  12  are connected, the PC  14  and the network  16  are connected. 
     Regarding a connection between the STA  10  and the AP  12 , at step ( 9 ), the STA  10  sends a message “Probe Request” to the AP  12 . At step ( 10 ), the AP  12  sends a message “Probe Response” back to the STA  10 . The exchange of these messages serves to check whether or not it is possible to communicate between the STA  10  and the AP  12 . At steps ( 11 ) and ( 12 ), the STA  10  and the AP  12  send messages “Authentication” each other to authenticate between the STA  10  and the AP  12 . After completing the authentication, the STA  10  sends a message “Association Request” to the AP  12  so as to connect thereto. At step ( 14 ), the AP  12  sends a message “Association Response” together with an IP address (172.16.40.10) back to the STA  10 . This IP address is the address that has been already allocated to the AP  12  by the DHCP server  18 . The STA  10  uses this IP address as its own address instead of the default IP address of the STA  10 . After completing the connection between the STA  10  and the AP  12  in the above manner, the STA  10  sends a message “DHCP RELEASE” to the PC  14  so as to invalidate the temporary IP address that has been allocated to the PC  14 . At step ( 16 ), the PC  14  sends the message “DHCP DISCOVER” again so as to acquire an IP address. This message “DHCP DISCOVER” is sent to the DHCP server  18  via the network  16  because the STA  10  and the AP  12  have the connection currently. According to the above-mentioned procedure that the AP  12  acquires the IP address from the DHCP server  18 ; at step ( 17 ), the DHCP server  18  sends the message “DHCP OFFER” back to the PC  14 . At step ( 18 ), the PC  14  sends the message “DHCP REQUEST” (IP address request) to the DHCP server  18 . At step ( 19 ), the DHCP server  18  sends the message “DHCP ACK” (reply for IP address request) back to the PC  14 , whereby the PC  14  acquires an IP address. 
     A description will now be given, with reference to flowcharts in  FIGS. 5 and 6 , of the above process between the STA  10  and the AP  12 .  FIG. 5  shows a flowchart illustrating a process regarding the STA mode, which corresponds to the process of the STA  10  in this case. On the other hand,  FIG. 6  shows a flowchart illustrating a process regarding the AP mode, which corresponds to the process of the AP  12  in this case. 
     A description will be given of a flowchart illustrating a process regarding the STA mode in  FIG. 5 . At step S 101 , it is determined whether or not a communication apparatus is in the STA mode. If the communication apparatus is not in the STA mode, the process proceeds to the AP mode at step S 102 . In this case, since the communication apparatus (the STA  10 ) is in the STA mode, the process proceeds to step S 103 . At step S 103 , it is determined whether or not the STA  10  is connected with an RJ  45  cable, and in other words, whether or not the STA  10  is connected with the physical layer. If not, this step is repeated until the STA  10  has a connection with the physical layer. If the connection is established, at step S 104 , it is determined whether or not an IP address acquisition setting of the STA  10  is set as an automatic setting. If not, the process proceeds to step S 105  of a manual setting. If the IP address acquisition setting is automatic, the STA  10  sends a message “DHCP RELEASE” for the following reason; When the PC  14  has already an IP address, it is likely to lack consistency with the STA  10 . Thus, the STA sends the message one more time in order to invalidate the IP address allocated to the PC  14 . Then, at step S 107 , the STA  10  executes a normal operation. The normal operation means that the STA  10  is in a state wherein the STA  10  is currently waiting for data or a message. This state continues until the STA  10  receives the message “DHCP DISCOVER” from the PC  14  that has received the message “DHCP RELEASE”. At step S 109 , when receiving the message “DHCP DISCOVER”, the STA  10  sends a message “DHCP OFFER” to the PC  14  and then is waiting for a message “DHCP REQUEST” from the PC  14 . At step S 110 , when receiving the message “DHCP REQUEST”, the STA  10  sends a message “DHCP ACK” to the PC  14 . Through the above process, the PC  14  acquires a temporary IP address. Then, the STA  10  proceeds to a process for connecting with the AP  12 . At step S 112 , the STA  10  sends a message “Probe Request” to the AP  12  and is waiting for a message “Probe Response” from the AP  12 . When receiving the message “Probe Response” at step S 113 , the STA  10  sends a message “Authentication” to authenticate and is waiting for the message “Authentication” from the AP  12 . When receiving the message “Authentication” at step S 115 , the STA  10  sends a message “Association Request” to connect with the AP  12  and is waiting for a message “Association Response”. At step S 117 , the STA  10  receives the message “Association Response” from the AP  12 . Through the above process, the STA  10  has the connection with the AP  12 . Now, the PC  14  is ready to communicate with the DHCP server  18  via the two communication apparatuses. In order to establish the connection with the DHCP server  18  properly, the STA to needs to let the PC  14  discard the temporary address that the STA  10  has already provided for the PC  14 . Thus, the STA  10  sends a message “DHCP RELEASE” to the PC  14  at step S 118  and executes the normal operation at step S 119 . 
     A description will now be given, with reference to a flowchart of  FIG. 6 , of a process regarding the AP mode. It should be noted that the process regarding the AP mode shown in  FIG. 6  is related to the connection between the AP  12  and the DHCP server  18 . 
     At step S 201 , it is determined whether or not a communication apparatus is in the AP mode. If the communication apparatus is not in the AP mode, the process proceeds to the STA mode at step S 202 . Since the communication apparatus (the AP  12 ) is in the AP mode in this case, the process proceeds to step S 203 . At step S 203 , it is determined whether or not the AP  12  is connected with an RJ  45  cable, and in other words, whether or not the AP  12  is connected with the physical layer. If not, this step is repeated until the AP  12  has a connection with the physical layer. If the connection is established, at step S 204 , it is determined whether or not an IP address acquisition setting of the AP  12  is set as an automatic setting. If not, the process proceeds to step S 205  of a manual setting. If the IP address acquisition setting is automatic, the AP  12  sends a message “DHCP DISCOVER” to the DHCP server  18  and is waiting for a message “DHCP OFFER”. When receiving the message “DHCP OFFER” at step S 207 , the AP  12  sends a message “DHCP REQUEST” to the DHCP server  18  at step S 208  and is waiting for a message “DHCP ACK” from the DHCP server  18 . When receiving the message “DHCP ACK” at step S 209 , the AP  12  proceeds to the normal operation at step S 210 . The process to allocate an IP address to the PC  14  by using the DHCP has been described under the case that it is impossible to connect with the communication apparatus  12  when the communication apparatus  10  receives an IP address request from the PC  14 . 
     A description will now be given, with reference to  FIG. 7 , of the case that it is possible to connect with the communication apparatus  12  when the communication apparatus  10  receives an IP address request from the PC  14 . 
     At the beginning, at step ( 1 ), the AP  12  sends a message “DHCP DISCOVER” (search) to search a DHCP server  18 . At step ( 2 ), the DHCP server  18  sends a message “DHCP OFFER” (reply for search) back to the AP  12  to show that the DHCP server  18  works. At step ( 3 ), the AP  12  sends a message “DHCP REQUEST” (IP address request) to the DHCP server  18 . At step ( 4 ), the DHCP server sends a message “DHCP ACK” (reply for IP address request) back to the AP  12 , and an IP address (for example, 172.16.40.10) is allocated to the AP  12  by the DHCP server  18 . On the other hand, when the DHCP server  18  cancels out an IP address that has been allocated to a DHCP client, the DHCP server  18  sends a message “DHCP RELEASE” to invalidate the IP address allocated. In the above manner, since the AP  12  has a connection with the network  16 , the above-mentioned some settings regarding the AP  12  can be manipulated from another PC via the network  16 . 
     A description will now be given of a correspondence between the PC  14  and the STA  10 . Suppose that the PC  14  has a setting in which the PC  14  acquires an IP address by the DHCP. Further suppose that an IP address (for example, 192.16.8.1.1) is allocated to the SAT  10  as a default and that the SAT  10  continues the IP address until the SAT knows an IP address of the AP  12  by establishing a connection with the AP  12 . Currently, the STA  10  and the AP  12  are assumed to have no connection each other. 
     In the above state, at step ( 5 ), the PC  14  sends a message “DHCP DISCOVER” to the STA  10 . At this time, the STA  10  attempts to connect to the AP  12 . At step ( 6 ), the STA  10  sends a message “Probe Request” to the AP  12 . At step ( 7 ), the AP  12  sends a message “Probe Responses” back to the STA  10 . The exchange of these messages serves to check whether or not it is possible to communicate between the STA  10  and the AP  12 . At steps ( 8 ) and ( 9 ), the STA  10  and the AP  12  send a message “Authentication” each other to authenticate. After completing the authentication, at step ( 10 ), the STA  10  sends a message “Association Request” to the AP  12  so as to connect thereto. At step ( 11 ), the AP  12  sends a message “Association Response” together with an IP address (172.16.40.10) back to the STA  10 . This IP address is the address that has been already allocated to the AP  12  by the DHCP server  18 . The STA  10  uses this IP address as its own address instead of the default address. In such a manner, after establishing the connection between the STA  10  and the AP  12 , the STA  10  sends a message “DHCP DISCOVER” that the STA  10  has already received from the PC  14 , and the message “DHCP DISCOVER” is sent to the DHCP server  18  via the network  16 . According to the above-mentioned procedure that the AP  12  acquires the IP address from the DHCP server  18 , at step ( 13 ), the DHCP server  18  sends the message “DHCP OFFER” to the PC  14 . At step ( 14 ), the PC  14  sends the message “DHCP REQUEST” (IP address request) to the DHCP server  18 . At step ( 15 ), the DHCP server  18  sends the message “DHCP ACK” (reply for IP address request) back to the PC  14 , whereby the PC  14  acquires an IP address. 
     A description will now be given, with reference to  FIG. 8 , of a roaming of the PC  14  possessing an IP address through the above process. The roaming in this description means that as shown in  FIG. 8 , the PC  14  and the STA  10  move and, in turn, connect with an AP  32  different from the AP  12  with which the PC  14  and the STA  10  have maintained a connection heretofore. In this case, the STA  10  needs to discard an IP address that the STA  10  has used and acquire a new IP address. In addition, if the AP  32  has a network address different from the AP  12 , the PC  14  also needs to acquire a new address. In the following, a description will be given, with reference to a flowchart of  FIG. 9 , of a roaming method including a way in which these two addresses are allocated. 
       FIG. 9  shows the flowchart illustrating a roaming process. At step S 301 , the STA  10  performs a normal operation such as a transit operation. At step S 302 , it is determined whether or not the AP  12  disappears. If not, the process returns to step S 301  and the STA  10  performs the normal operation. If the AP  12  disappears, the STA  10  sends a message “Probe Request” at step S 303  and is looking for another AP. Regarding the disappearing determination, when the STA  10  cannot receive a beacon propagated by the AP  12 , it is supposed that the AP  12  disappears. After sending the message “Probe Request”, the STA  10  is waiting for receiving a message “Probe Response”. At step S 304 , if the STA  10  cannot receive the message “Probe Response”, the STA  10  sends the message “Probe Request” again. If the STA  10  can receive the message “Probe Response”, the STA  10  sends a message “ReAssociation Request” (reconnect request) at step S 305 . At step S 306 , if the STA  10  receives a message “ReAssociation Response”, the STA  10  returns to step S 301 . If not, it is determined whether or not the STA  10  receives a message “DeAuthentication”. The message “DeAuthentication” means an authentication denial, which is sent in a case that the STA  10  and the AP  32 , which is expected to be a substitute for the AP  12 , do not authenticate each other. If the STA  10  does not receive the message “DeAuthentication”, the STA returns to step S 306 . If the STA  10  receives the message “DeAuthentication”, the STA  10  sends a message “Authentication” for requesting the authentication to the AP  32  at step S 308  and is waiting for a message “Authentication” meaning an acceptance of the authentication. When receiving the message “Authentication” at step S 309 , the STA  10  sends a message “ReAssociation Requests” (reconnect request) to the AP  32  at step S 310  and is waiting for a message “ReAssociation Response” (reconnect reply). Then, the STA  10  receives the message “ReAssociation Response” at step S 311 . 
     The message “ReAssociation Response” includes a new IP address that the AP  32  allocates to the STA  10 . When the IP address is set to the STA  10 , the roaming process for the STA  10  is completed. As mentioned above, if the AP  32  has a network address different from the AP  12 , the PC  14  also needs to acquire a new address. Accordingly, the STA  10  sends the message “DHCP RELEASE” shown in  FIG. 4  to the PC  14  in order to let the PC  14  discard an IP address that the PC  14  currently maintains. The PC  14  sends the message “DHCP DISCOVER” to the DHCP server  18  in order to acquire a new IF address. The message “DHCP DISCOVER” is sent to the DHCP server  18  via the STA  10  and the AP  32 , because the STA  10  and the AP  32  have already established a connection each other through the above-mentioned process. The PC  14  acquires the IP address in accordance with the procedure of the DHCP shown in  FIG. 4 . 
     Under the above-mentioned embodiment of the present invention, the operations ( 9 ) through ( 14 ) in  FIG. 4 , the operations S 112  through S 117  in  FIG. 5 , and the operations ( 6 ) through ( 11 ) in  FIG. 6  correspond to a means to establish a communication path. Also, the operation ( 6 ) in  FIG. 4  and the operation S 109  in  FIG. 5  correspond to a means to provide a temporary IP address. The operation ( 15 ) in  FIG. 4  and the operation S 118  in  FIG. 5  correspond to a mean to discard the temporary IP address. The operations S 303  through S 311  correspond to a reconnect request at a roaming. 
     The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention. 
     The present application is based on Japanese priority application No. 2001-351469 filed Nov. 16, 2001, the entire contents of which are hereby incorporated by reference.