Abstract:
The object of the present invention is to provide a DNS server and terminals both of which are capable of sending and receiving communication by means of a small number of IP addresses. In the case where a firewall is not provided, when the DNS server receives an inquiry of a FQDN, it sends a broad cast frame for searching a terminal to which an IP address is assigned via all the network devices. When a terminal monitors the broad cast frame and finds the FQDN to be its own FQDN, it returns the uni-cast frame to the DNS server. When the DNS server receives the uni-cast frame, it makes a look-up table and a reverse look-up table. As a result, the terminal can receive the communication by an ordinary method. In the case where the firewall is provided, by processing an IP address conversion between the external IP address and the internal address by the use of an IP address conversion means NAT in addition to making the look-up table and the reverse look-up table, the terminal can receive communication.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a DNS (Domain Name System) server, a DHCP (Dynamic Host Configuration Protocol) server, a terminal and a communication system of an Internet capable of supporting a large number of terminals by means of a small number of IP addresses by dynamically assigning a limited number of IP addresses only to now communicating terminals.  
         [0003]     2. Description of the Related Art  
         [0004]     Among conventional Internets, as shown in  FIG. 11 , is a system for connecting a DNS server  50  to a plurality of terminals (personal computer terminals)  52   a ,  52   b ,  52   c , - - - ,  52   n  through a dedicated line or LAN/MAN  51 . Usually, in the DNS server  50  are stored a look-up table for storing FQDNs (fully qualified domain name-host name) as shown in  FIG. 12A  and a reverse look-up table as shown in  FIG. 12B . When a inquiry about a FQDN is made of a DNS server from a terminal (for example,  52   a ) connected to the DNS server  50  or an external terminal connected to the Internet in the case where the terminal is to be connected to a terminal corresponding to the FQDN, the DNS server  50  finds an IP address from the FQDN with reference to the look-up table, whereby the terminal is connected to the terminal corresponding to the FQDN. In some case, the DNS server  50  finds a FQDN from the IP address with reference to the reverse look-up table.  
         [0005]     In the conventional system described above, there is presented a problem that each of the terminals  52   a ,  52   b ,  52   c , - - - ,  52   n  needs to have an address, which leads to the depletion of the IP addresses managed by the DNS server  50 .  
         [0006]     In order to solve this problem, for example, a system employing the DHCP server  60  shown in  FIG. 13  is proposed. The DHCP server  60  has a function of assigning an IP address and is connected to, for example, a public exchange network  61  through 10 lines and the public exchange network  61  is connected to, for example, 100 terminals  62   a  to  62   n . Usually, each of the terminals  62   a  to  62   n  is not assigned an IP addresses and when the terminal sends IP communications, it requires the DHCP server  60  to assign an IP address thereto and is assigned the IP address by the DHCP server  60 .  
         [0007]     For example, when a terminal  62   a  is going to conduct the IP communications, the DHCP server  60  assigns an IP address to the terminal  62   a . As a result, the terminal  62   a  can start conducting the IP communications with the other terminals. When the terminal does not conduct the IP communications even after a predetermined time elapses after the completion of this communications, the terminal  62   a  returns the IP address to the DHCP server  60  and returns to the initial state where it has no IP address.  
         [0008]     According to this conventional system, it is essential only that the DHCP server  60  assigns the IP address to the terminal which requires the IP communications every time it requires the IP communications, and hence the DHCP server  60  can manage the terminals  62   a  to  62   n  with a small number of IP addresses. Also, this can save the number of ports of the DHCP server  60 .  
         [0009]     In the conventional system described above, however, an IP address is not previously assigned to each of the terminals  62   a  to  62   n , and hence the terminals  62   a  to  62   n  can not receive communication. In other words, the conventional system has a problem that it is designed specifically for sending communication.  
       SUMMARY OF THE INVENTION  
       [0010]     The object of the present invention is to provide a DNS server, a DHCP server, a terminal and a communication system which can send and receive communication by means of a small number of IP addresses.  
         [0011]     In order to achieve the object, the present invention is firstly characterized in that a DNS server comprises means for holding a table in which FQDNs of terminals managed by the DNS server are registered in advance and the list of IP addresses to be broad cast to the outside; address conversion means for converting one of the IP addresses in the list to be broad cast to the outside into one of an address of a data link layer and a network layer in the sense of OSI communications between the terminals managed by the DNS server, when the DNS server receives an inquiry of the FQDN; and registration means for registering the IP addresses in the table in correspondence with the FQDNs, wherein the terminal required to receive communication can receive it. According to the invention, in the case where a fire wall is not provided, the terminal can receive communications through an Internet from the external terminal.  
         [0012]     The present invention is secondly characterized in that a communication system comprises an external DNS server provided on a fire wall for managing a correspondence relationship between the FQDN and the IP address of a terminal both of which are to be broad cast to the outside; IP address conversion means NAT provided on the fire wall: and an internal DNS server provided in the firewall and for managing a correspondence relationship between the FQDN and the IP address of a terminal, both of which are used in the fire wall, wherein when there is an inquiry from inside the fire wall, the internal DNS server returns an IP address found from the correspondences relationship between the FQDN and the IP address of a terminal, both of which are used in the fire wall, wherein when there is an inquiry from outside the fire wall, the external DNS server inquires of the internal DNS server an internal IP address corresponding to the FQDN and corresponds one of the IP addresses in the external broad cast list to the internal IP address of the terminal and registers the external IP address in correspondence with the FQDN, and wherein the IP address conversion means NAT converts an IP address (external IP address) destination of a packet passing the fire wall from outside to inside into the internal IP address, and a source IP address (internal IP address) of a packet passing the fire wall from inside to outside into the IP address of the NAT. According to the invention, in the case where the fire wall is provided, the terminal can receive communication from the external terminal.  
         [0013]     According to the first and second feature of the invention, IP address are dynamically assigned only to now communicating terminals. Therefore, this makes it possible to eliminate a problem of depletion of the IP address managed by the DNS server and to make the terminal receive and send communication.  
         [0014]     The present invention is thirdly characterized in that a terminal corresponding to the FQDN of a broad cast frame sent by the DNS server initializes a network device receiving the broad cast frame by the IP address and the net mask given by the broad cast frame. According to the invention, the terminal which is required of receiving is dynamically assigned IP address and can receive communication. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a circuit diagram to show the schematic system configuration of one preferred embodiment, in the case where a fire wall is not provided, in accordance with the present invention;  
         [0016]      FIG. 2  is a conceptional view of a look-up table formed in a DNS server;  
         [0017]      FIG. 3  is an illustration of a hierarchical structure of LAN communications;  
         [0018]      FIG. 4  is an illustration of one example of a broad cast frame;  
         [0019]      FIG. 5  is an illustration of one example of a uni-cast frame;  
         [0020]      FIG. 6  is a timing chart to show a communication receiving operation of one preferred embodiment in the case where a fire wall is not provided;  
         [0021]      FIG. 7  is a timing chart to show a communication sending operation of one preferred embodiment in the case where a fire wall is not provided;  
         [0022]      FIG. 8  is a circuit diagram to show a schematic system configuration of one preferred embodiment in the case where a fire wall is provided;  
         [0023]      FIG. 9  is a timing chart to show a communication receiving operation of one preferred embodiment in the case where a fire wall is provided;  
         [0024]      FIG. 10  is a timing chart to show a communication sending operation of one preferred embodiment in the case where a fire wall is provided;  
         [0025]      FIG. 11  is a circuit diagram to show a conventional system configuration;  
         [0026]      FIG. 12A  is a conceptional view of a look-up table and  FIG. 12B  is a conceptional view of a reverse look-up table; and  
         [0027]      FIG. 13  is a circuit diagram to show another conventional system configuration. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]     The present invention will be hereinafter described in detail with reference to the accompanying drawings.  FIG. 1  to  FIG. 7  show one preferred embodiment in accordance with the present invention in the case where a fire wall is not provided between terminals and an Internet, which is typified by a dial-up service of a provider.  
         [0029]     As shown in  FIG. 1 , assume that the DNS server  1  of the present embodiment is connected to terminals (for example, personal computer) pc 1 , pc 10 , pc 15 , pc 20  and the like and a DHCP server  4  through a network LAN  2 , for example, and that the LAN  2  is connected to an Internet through a router  3 . Also, assume that FQDNs are previously assigned to the terminals pc 1 , pc 10 , pc 15 , pc 20  and the like. Further, assume that each of these terminals pc 1  to pc 20  always gets a system up and running, which monitors the broad cast frame (for example, the broad cast frame of an Ethernet) of a data link layer of a network device Ndev connected to a network and detects a broad cast frame having a specific protocol ID for assigning an IP address described below.  
         [0030]      FIG. 2  shows one example of a look-up table formed in the DNS server  1 , and in this look-up table  5  are stored the FQDNs of the terminals connected to the LAN  2 , wherein IP addresses corresponding to the respective FQDNs are not determined. Also, in the DNS server  1  are stored a set of available IP addresses and a set of not-yet-used IP addresses. Further, as shown in  FIG. 3 , in the LAN  2  is used an IP protocol on an Ethernet. Here,  FIG. 3  is an illustration of a hierarchical structure of the communications of the LAN  2 .  
         [0031]     An operation when the terminal pc 1  receives communication in the above system, for example, will be described with reference to  FIG. 1 ,  FIG. 4  to  FIG. 6 . Here,  FIG. 4  is a system configuration of a broad cast frame,  FIG. 5  is a system configuration of a uni-cast frame, and  FIG. 6  is a timing chart to show the outline of an operation of the present embodiment.  
         [0032]     Now, as shown in  FIG. 6 , assume that an inquiry about the FQDN, for example, “pc 1 .ncs.kddlabs.co.jp”, of a terminal pc 1  in the domain managed by the DNS server  1  is made of the DNS server  1  through an Internet from an external terminal X, the DNS server  1  searches a not-yet-used IP address and if there is not an unassigned IP address, the DNS server  1  answers that there is not a terminal to the inquiry to thereby keep conformity in the ordinary protocol with respect to the DNS server.  
         [0033]     On the other hand, if there is a not-yet-used IP address, the DNS server  1  sends a broad cast frame for searching a terminal, to which an IP address is to be assigned, by the standards of Ethernet, through all of the network devices Ndev (for example, Ndev  2  in  FIG. 1 ) connected to the DNS server  1 . In this broad cast frame, as shown in  FIG. 4 , are included an ID  4   a  to identify a frame relating to this protocol, the MAC address  4   b  of DNS server  1  from which communication is sent, an IP address  4   c  of the DNS server  1  from which communication is sent, an IP address  4   d  to be assigned to the terminal pc 1 , a net mask  4   e  to be assigned to the terminal pc 1 , and the FQDN  4   f  of the terminal pc 1 .  
         [0034]     When the broad cast frame in  FIG. 4  is sent from the DNS server  1 , the terminal pc 1 , for example, to which “pc 1 .nes.kddlabs.co.jp” is assigned, answers to this and initializes the network device Ndev receiving the broad cast frame by the assigned IP address  4   d  and the net mask  4   e . Thereafter, the terminal pc 1  returns a uni-cast frame including the following information to the MAC address  4   b  of the DNS server  1  from which the broad cast frame in  FIG. 4  is sent. That is, the terminal pc 1  assigns the above-mentioned IP address  4   d  to itself and returns the uni-cast frame, which is an ACK signal, to the address  4   b  which receives the uni-cast frame, that is, to the DNS server  1 .  
         [0035]     In the above-mentioned uni-cast frame, as shown in  FIG. 5 , are included ID  5   a  to identify a frame relating to this protocol, the MAC address  5   b  of the DNS server  1 , an IP address  5   c  of the DNS server  1 , an IP address  5   d  to be assigned to the terminal pc 1 , a net mask  5   e  to be assigned to the terminal pc 1 , and the FQDN  5   f  of the terminal pc 1 .  
         [0036]     When the DNS server  1  receives this uni-cast frame, it describes the assigned IP address, for example, “133.128.8.15” in the IP address column, to which an IP address has been not yet assigned, of the look-up table  5  (see  FIG. 2 ) and makes a reverse look-up table (not shown) at the same time (see step S 1  in  FIG. 6 ), and thereafter functions as an ordinary DNS server. That is, since the IP address is assigned to the communication receiving terminal pc 1 , the communication sending terminal establishes and conducts communications with the terminal pc 1  by the ordinary procedure.  
         [0037]     Next, the terminal pc 1  monitors the state of itself after the communications start (S 2 ) and when the terminal pc 1  detects that the terminal pc 1  is not used for 15 minutes, for example, the terminal pc 1  sends an IP address return command (UDP packet) for returning the assigned IP address to the DNS sever  1 . When the DNS server  1  receives the IP address return command, it returns a signal ACK  1  to the terminal pc 1 . When the terminal pc 1  receives the ACK  1 , it returns a signal ACK  2  to the DNS server  1  and resets the network device Ndev corresponding thereto (S 3 ). Here, when the terminal pc 1  does not receive the above-mentioned ACK  1  within a predetermined period after it sends the IP address return command, it sends the IP address return command to the DNS server  1  once more.  
         [0038]     When the DNS server  1  receives the above-mentioned ACK  2 , it deletes the IP address assigned to the terminal pc 1  from the look-up table  5  and the reverse look-up table (S 4 ) and registers the IP address once more in a set of not-yet-used IP addresses.  
         [0039]     In this manner, according to the present embodiment, it is possible to receive communications through the Internet from the external terminal.  
         [0040]     In this connection, the difference between the case where the terminal pc 1  sends communication, for example, in the above-mentioned system and the conventional DHCP server will be described with reference to  FIG. 1 ,  FIG. 2  and  FIG. 7 . Here,  FIG. 7  is a timing chart to show the outline of the operation of the present embodiment. The operation between the terminal pc 1  and the like and the DHCP server  4  is similar to that with respect to the conventional DHCP server described in a RFC1541 and hence its detailed description will be omitted.  
         [0041]     When the terminal pc 1  sends a signal DHCPDISCOVER, the DHCP server  4  receiving the signal DHCPDISCOVER asks the DNS server  1  whether the DNS server  1  has a not-yet-used IP address or not in the state where it determines the setting of the terminal (S 71 ). If the DNS server  1  has an unassigned IP address, it returns a signal ACK  71  to this inquiry to the DHCP server  4  and the DHCP server  4  returns a signal DHCPOFFER to the terminal pc 1  by the use of the IP address assigned by the DNS server  1 . When the terminal pc 1  selects the assigned setting information (S 72 ), it puts the FQDN previously set to itself in the Host Name option of a DHCPREQUEST and returns the DHCPREQUEST to the DHCP server  4 . The DHCP server  4  receiving the DHCPREQUEST sends an IP address register command to register the relationship between the IP address and the FQDN. When the DNS server  1  receives the IP address register command, it returns a signal ACK  72  to the DHCP server  4 . When the DHCP server  4  receives the ACK  72 , it returns a signal ACK  73  to the DNS server  1  and returns the corresponding terminal setting DHCPACK to the terminal pc 1  (S 73 ). Here, when the DHCP server  4  does not receive the above-mentioned ACK  72  within a predetermined time after it sends the above-mentioned IP address register command, it sends the IP address register command to the DNS server  1  once more.  
         [0042]     When the DNS server  1  receives the above-mentioned ACK  73 , it describes the assigned IP address, for example, “133.128.8.15”, in the IP address column, which is not yet used, of the look-up table  5  ( FIG. 2 ) and makes a reverse look-up table (not shown) at the same time (S 74 ) and thereafter functions as an ordinary DNS server. That is, since the IP address is assigned to the terminal pc 1 , which is the communication sending terminal, the communication sending terminal can establish and conduct communications with the terminal pc 1  by the ordinary procedure.  
         [0043]     Next, after the communications start, when the DHCP server  4  receives a request of returning the IP address by DHCPRELEASE from the terminal pc 1  (S 75 ), the DHCP server  4  sends an IP address return command (UDP packet) for returning the IP address assigned to the pc 1  to the DNS server  1 . When the DNS server  1  receives the IP address return command, it returns a signal ACK  74  to the DHCP server  4 . When the DHCP server  4  receives the ACK  74 , it returns a signal ACK  75  to the DNS server  1  (S 76 ). Here, when the DHCP server  4  does not receive the ACK  74  within a predetermined time after it sends the IP address return command, it sends the IP address return command to the DNS server  1  once more.  
         [0044]     When the DNS server  1  receives the ACK  75 , it deletes the IP address assigned to the terminal pc 1  from the look-up table  5  and the reverse look-up table (S 77 ) and registers the IP address in a set of unassigned IP addresses.  
         [0045]     In this connection, by entering  0  for the existing time (TTL) of the data when the DNS server  1  registers an external IP address corresponding to the FQDN in the table, it is possible to prevent a DNS server on an external Internet from caching the registered information of the FQDN and the IP address.  
         [0046]     Next, the preferred embodiment in accordance with the present invention in the case where a fire wall  13  is provided between the terminals and the Internet such as a corporate LAN will be described in detail in the following.  FIG. 8  is a block diagram to show the schematic system configuration of one preferred embodiment in accordance with the present invention,  FIG. 9  is a timing chart to show a communication receiving operation, and  FIG. 10  is a timing chart to show a communication sending operation.  
         [0047]     As shown in  FIG. 8 , assume that the external DNS server  11  of the present embodiment is connected to an Internet and an internal network, for example, a LAN  12  via a fire wall  13 . Assume that the terminals (for example, personal computers) pc 1 , pc 10 , pc 15 , pc 20 , and the like, and an internal DNS server  15  are connected to the LAN  12  to establish mutual communications with an Internet outside the fire wall by means of a network address translator (NAT)  14 . Assume that the FQDNs of the terminals pc 1 , pc 10 , pc 15 , pc 20 , and the like are assigned in advance to the external DNS server  11 . Assume that internal IP addresses (private addresses) as well as the FQDNs of the terminals pc 1 , pc 10 , pc 15 , pc 20 , and the like are already registered in the internal DNS server  15  for managing mutual communications between the terminals in the internal network and that, in the communications in the internal LAN, it is possible to refer to the internal IP address from the FQDN and the FQDN from the internal IP address by means of the internal DNS server  15 .  
         [0048]      FIG. 2  shows one example of a look-up table formed in the external DNS server  11  and, in this look-up table  5 , FQDNs connected to the LAN  12  are registered and IP addresses corresponding to the FQDNs are not determined. Also, in the external DNS server  11  are registered a set of available IP addresses and a set of now not-yet-used IP addresses.  
         [0049]     The operation when the terminal pc 1  receives communication in the above-mentioned system, for example, will be described with reference to  FIG. 8  and  FIG. 9 .  
         [0050]     Now, as shown in  FIG. 9 , when an external terminal X asks an external DNS server  11 , via an Internet, about the FQDN of the terminal pc 1  of the domain managed by the external DNS server  11 , for example “pc 1 .nes.kddlabs.co.jp”, the external DNS server  11  searches an not-yet-used IP address and if it does not find an not-yet-used IP address, it answers the external terminal X that there is no terminal corresponding to the inquiry, whereby conformity on the protocol to the ordinary DNS server can be held.  
         [0051]     On the other hand, if there is an not-yet-used IP address, the external DNS sever  11  asks the internal DNS server  15  about the internal IP address corresponding to the FQDN (see a step S 91  in  FIG. 9 ). If the internal DNS server  15  does not find the internal IP address corresponding thereto, it answers the external DNS server  11  that there is no terminal corresponding to the inquiry, whereby conformity on the protocol to the ordinary DNS server can be held.  
         [0052]     If the external DNS server  11  receives the corresponding internal IP address from the internal DNS server  15 , it sends an IP address conversion command to a network address translator (NAT)  14  to direct the address conversion between the external IP address, which is to be newly selected and assigned from among not-yet-used IP addresses, for example, “133.128.8.15”, and the internal IP address (S 92 ). When the NAT  14  receives the IP address conversion command, it starts address conversion between the directed external and internal IP addresses (S 93 ) and returns a signal ACK  91  to the external DNS server  11 . When the external DNS server  11  receives the ACK  91 , it describes the external IP address in the IP address column, which is not yet determined, of the look-up table  5  ( FIG. 2 ) and makes a reverse look-up table (not shown) at the same time (S 94 ) and thereafter functions as an ordinary DNS server. That is, since the external IP address of the fire wall is assigned to the terminal pc 1 , which is the communication receiving terminal, a communication sending terminal outside the fire wall can establish and conduct communications with the terminal pc 1  by the ordinary procedure.  
         [0053]     Next, after the communications start, the NAT  14  monitors the state of the communications between the terminals subjected to the address conversion (S 95 ), and when it finds that the NAT  14  is disconnected from the terminal, it sends an IP address return command for returning the external IP address to the external DNS server  11 . When the external DNS server  11  receives the IP address return command, it returns a signal ACK  92  to the NAT  14 . When the NAT  14  receives the ACK  92 , it returns a signal ACK  93  to the external DNS server  11  to stop the address conversion to the corresponding external IP address (S 96 ). Here, when the NAT  14  does not receive the ACK  92  within a predetermined time after it sends the IP address return command, it sends the IP address return command to the external DNS server  11  once more.  
         [0054]     When the external DNS server  11  receives the ACK  93 , it deletes the IP address assigned to the terminal pc 1  from the look-up table  5  and the reverse look-up table (see step S 97  in  FIG. 9 ) and registers the IP address again in the set of not-yet-used IP addresses.  
         [0055]     Next, the operation when the terminal pc 1  sends communication in the above-mentioned system, for example, will be described with reference to  FIG. 8  and  FIG. 10 .  
         [0056]     For example, when the terminal pc 1  sends communication, the terminal pc 1  outputs an IP packet for an external terminal X. The NAT  14 , unlike the conventional NAT  14 , outputs an external IP address request command including the internal IP address of the terminal pc 1  to the external DNS server  11  every time it receives the IP packet for the external terminal X. When the external DNS server  11  receives the external IP address request command, it searches an not-yet-used IP address and if it does not find the not-yet-used IP address, it returns non-permission of the address conversion (NACK) to the NAT  14 . When the NAT  14  receives the NACK, it does not permit the terminal pc 1  to be connected to an external terminal.  
         [0057]     On the other hand, if the external DNS server  11  finds the not-yet-used IP address, it asks the internal DNS server  15  about the FQDN corresponding to the internal IP address and gets the FQDN. When the internal DNS server  15  does not succeed in getting the FQDN, it returns the non-permission of the address conversion (NACK). On the other hand, when the internal DNS server  15  succeeds in getting the FQDN, it sends the gotten FQDN of the terminal pc 1  to the external DNS server  11 . The external DNS server  11  returns to the NAT  14  an IP address conversion command for directing the address conversion between the external IP address to be newly selected and assigned from among the not-yet-used IP addresses, for example, “133.128.8.15”, and the internal IP address. When the NAT  14  receives the IP address conversion command, it starts the mutual IP address conversion between the directed IP addresses (S 101 ) and returns a signal ACK  101  to the external DNS server  11 . When the external DNS server  11  receives the ACK  101 , it describes the external IP address in the IP address column, which is not yet determined, of the look-up table  5  (see  FIG. 2 ) and makes a reverse look-up table (not shown) at the same time (S 102 ). That is, since the external IP address of the fire wall is assigned to the terminal pc 1 , which is the communication sending terminal, the terminal pc 1  can establish and conduct communications with a communication receiving terminal outside the fire wall by the ordinary procedure.  
         [0058]     Next, after the communications start, the NAT  14  monitors the state of the communications between the terminals subjected to the address conversion (S 103 ), and when it finds that the NAT  14  is disconnected from the terminal, it sends an IP address return command for returning the external IP address to the external DNS server  11 . When the external DNS server  11  receives the IP address return command, it returns a signal ACK  102  to the NAT  14 . When the NAT  14  receives the ACK  102 , it returns ACK  103  to the external DNS server  11  to stop the address conversion to the corresponding external IP address (S 104 ). Here, when the NAT  14  does not receive the ACK  102  within a predetermined time after it sends the IP address return command, it sends the IP address return command to the external DNS server  11  once more.  
         [0059]     When the external DNS server  11  receives the ACK  103 , it deletes the IP address assigned to the terminal pc 1  from the look-up table and the reverse look-up table (S 105 ) and registers the IP address again in the set of not-yet-used IP addresses.  
         [0060]     In this connection, by entering  0  for the existing time (TTL) of the data when the DNS server  11  registers an external IP address corresponding to the FQDN in the table, it is possible to prevent a DNS server on an external Internet from caching the registered information of the FQDN and the external IP address.  
         [0061]     As described above, according to the present embodiment, it is possible not only to send communication from one terminal to the other terminal to establish communications between them but also to receive communication sent from the other terminal.  
         [0062]     In this connection, while the present invention has been described by the use of the system in which the DNS server is connected to the terminals by means of the LAN in the above-mentioned embodiment, it is not intended to limit the present invention to this embodiment, but it can be also applied to a system in which a DNS server is connected to terminals by means of a public telephone line, a dedicated line, or the like.  
         [0063]     As is evident from the above description, when the DNS server in accordance with the present invention receives an inquiry of a FQDN from an external Internet terminal, in the case where a fire wall is not provided, it sends a broad cast frame for searching a terminal to which an IP address is assigned via all network devices connected to the DNS server, and when it receives an answer to the broad cast frame, it registers an external IP address in correspondence with the FQDN in a look-up table and a reverse look-up table. On the other hand, in the case where the fire wall is provided, the DNS server in accordance with the present invention gets the internal IP address (private address) corresponding to the FQDN from an internal DNS server and directs the address conversion between the external IP address and the internal IP address to a NAT and then registers the external IP address in the look-up table and the reverse look-up table in correspondence with the FQDN. Therefore, this makes it possible to eliminate a problem of depletion of the IP addresses managed by the DNS server and to make the terminal receive communication.  
         [0064]     Further, when an internal terminal sends communication to an external Internet, in the case where a fire wall is not provided, a DHCP server requires a DNS server to permit an address assignment, while in the case where the fire wall is provided, a NAT requires the DNS server to permit the address assignment. Therefore, this makes it possible to realize the present invention without largely changing the existing DHCP or NAT.  
         [0065]     Still further, in the case where the fire wall is not provided, a terminal corresponding to the FQDN of the broad cast frame sent by the DNS server is temporarily assigned an IP address by the DNS server and hence, when it receives communication, it can receive the communication by an ordinary procedure.