Abstract:
Disclosed is a packet-switched communication apparatus (DNS proxy server) receiving and forwarding messages which are used to obtain an address corresponding to a domain name assigned to a device connected to a network from the domain name and to obtain such a domain name from the address. The communication apparatus comprises mapping retaining means for retaining mappings between network-dependent parts of addresses which are used in a first network and address translators; address translator selecting means for selecting an address translators, based on an address included in a received packet; address obtaining means for obtaining a substitutive address to replace the address included in the received packet from the selected address translator; and address replacement means for replacing the address included in the received packet by using the obtained substitutive address.

Description:
FIELD OF THE INVENTION 
     The present invention relates to packet-switched communication. More particularly, the invention relates to a packet-switched communication apparatus (DNS proxy server) receiving and forwarding messages which are used to obtain an address corresponding to a domain name assigned to a device connected to a network from the domain name and to obtain such a domain name from the address. 
     BACKGROUND OF THE INVENTION 
     Protocol translation is one technique to realize communication between two terminals under circumstances where a network to which one terminal is attached and another network to which the other terminal is attached run on different communication protocols. 
     For instance, concerning the Internet Protocol (IP) for use on the Internet, the Internet Protocol version 4 (IPv4) is now in common use throughout the world, but there is a concern about depletion of available addresses. To solve this problem, the Internet Protocol version 6 (IPv6) has been proposed and has reached the stage of its implementation. However, for all existing systems on the Internet, concurrent transition from IPv4 to IPv6 is practically impossible. Accordingly, methods of IP packet protocol translation for interconnecting a node on a network using IPv4 and a node on a network using IPv6 and enabling communication across these networks have been proposed. As concrete examples of these conversion methods, the following are known: “NAT-PT” described in Request For Comments (RFC) 2765 and RFC 2766 published by the Internet Engineering Task Force (IETF); “SOCKS64” described in RFC3089; “transport relay” described in RFC3142; and so forth. 
     In any of these techniques, IPv4/IPv6 address translation rules for mutual translation between IPv4 addresses and IPv6 addresses must have been created and held beforehand. This translation rules may be set statically in advance or may be created dynamically each time a communication path established. In the latter case, a name resolution technique in the Domain Name System (DNS) is used to trigger generation of a translation rule. 
     The DNS is a system to search out attributes (resource record) associated with a fully qualified domain name (FQDN), which is assigned to each device connected to a network and uniquely defined all over the world. Generally, the DNS is used to obtain an IP address from a domain name. This process of obtaining an IP address from a domain name is called name resolution. Nowadays, most applications taking advantage of the Internet obtain the IP address of a corresponding node to which to communicate with, using the DNS. 
     When IPv4/IPv6 translation is required, this DNS is used and a DNS message which must be transmitted to initiate communication is always monitored, and an IP address translation rule is created, triggered by a name resolution request message. Moreover, the IP address in a name resolution response message is replaced by an IP address determined, according to the created address translation rule, and the name resolution response message is sent back to the requester. To provide these functions, a DNS proxy server linked with an IPv4/IPv6 translator is employed between terminals (clients) which may send a name resolution request and a DNS server to which a query is sent. 
     A practical example of this operation for a scenario where communication is initiated from an IPv6 client to an IPv6 client will be discussed below. 
     First, the IPv6 client which is an originating terminal sends a query about the IPv6 address of the receiving terminal (client) to the DNS proxy server for name resolution. Upon having received this query, the DNS proxy server forwards this query to another DNS server and receives notification of the address of the receiving terminal (client) from the DNS server in the response to the query. Here, if the notified address is an IPv4 address, the DNS proxy server replaces the IPv4 address in the response message by a temporary IPv6 address and returns this IPv6 address to the IPv6 client. At this time, the DNS proxy server requests the IPv4/IPv6 translator to create an address translation rule that maps the IPv4 address before being replaced to the temporary IPv6 that replaces the IPv4 address. The created address translation rule is held in a table (address translation table) in the IPv4/IPv6 translator. 
     The originating IPv6 client transmits an IPv6 packet to the temporary IPv6 address of the receiving terminal, which was notified as the result of the name resolution. The source address of this IPv6 packet is the IPv6 address of the originating client. The IPv4/IPv6 translator receives this IPv6 packet, refers to the address translation table and searches for the IPv4 address associated with the destination IPv6 address of the IPv6 packet. Because the mapping between the IPv4 address and the temporary IPv6 address created at the time of the name resolution is held in the address translation table, the IPv4 address of the receiving terminal (client) can be obtained. 
     Then, the IPv4/IPv6 translator refers to the address translation table and searches for the IPv4 address corresponding to the source IPv6 address of the IPv6 packet. However, an address translation rule for the source address has not yet been created at this point of time and, therefore, the target IPv4 address cannot be obtained. The IPv4/IPv6 translator newly assigns the originating client terminal a temporary IPv4 address mapped to its IPv6 address and registers this address translation rule into the address translation table. Then, the source and destination IPv4 addresses are obtained. The IPv6 packet is translated into an IPv4 packet having the source and destination IPv4 addresses which replaced the corresponding IPv6 addresses and the IPv4 packet is transmitted to the receiving destination. 
     Subsequent packets to be transmitted between both clients (terminals) are translated-between IPv6 and IPv4, according to the address translation rules, as both source and destination address translation rules have been registered into the address translation table. Because the address translation rules dynamically generated for a communication are temporary, they are discarded upon the elapse of a predetermined time after the termination of the communication. 
     While the communication initiated from the IPv6 client to the IPv4 client has been discussed in the foregoing example, address translation rules are generated and communication is performed by way of address translation in the same procedure as described above for other situations where the IPv4 client initiates communication to the IPv6 client, where communication is performed between IPv4 clients, but requiring address translation (for example, communication across two IPv4 private networks where duplicated addresses may exist), where communication is performed, using communication protocols other than IP, and so on (for example, refer to Japanese Patent Document Cited 1). 
     For mapping between an IPv4 address and an IPv6 address, using the above-mentioned DNS, the IP address included in a name resolution response message must be replaced by another one. This IP address is included in a payload following an IP header, not in the IP header. 
     In the DNS, in reverse to obtaining an IP address by specifying the domain name of a corresponding node to which to communicate with, a process called “reverse lookup” is also be performed, in which, by specifying an IP address, the domain name of the corresponding node assigned the IP address is obtained. This reverse lookup is mainly used for e-mail, server-to-client communication, and the like. 
     Particularly, in a network topology where an IPv4 network and an IPv6 network are interconnected by protocol translation means, e-mail is a practically essential service. As transition to IPv6 is in progress in access networks, a further increase in demand for connecting, for example, an IPv6 client on a user terminal to an existing server on an IPv4 network is expected in future. Accordingly, in order to implement these services, it is essential to perform a reverse lookup through the DNS to look up of an IPv4 address from the IPv6 network side and to lookup of an IPv6 address from the IPv4 network side. 
     In the communication environment requiring IP address translation, a client terminal communicates with a corresponding client terminal, regarding a temporary IP address assigned to the corresponding client by the address translator as the IP address of the corresponding client. Accordingly, when the client terminal sends a reverse lookup query, the client would specify the temporary IP address of the corresponding client and make the query about its domain name. On the other hand, a DNS server holds a mapping between the actual IP address of a client terminal and its domain name. In order to properly obtain the domain name of the corresponding client, a query about the actual IP address of the client terminal must be sent to the DNS server. Thus, to perform a reverse lookup through the DNS in a network topology involving an IPv4 network and an IPv6 network, the temporary address of the corresponding client specified for query from the client terminal that is a reverse lookup requester must be translated into its actual address to be recognized by the DNS server. 
     In a packet for a reverse lookup through the DNS, specifically, an FQDN for reverse lookup corresponding to an IPv4 address or IPv6 address which serves as a query key is stored in the payload portion of the packet, following the IP header. Accordingly, the FQDN for reverse lookup in the payload portion must be translated for IP address translation. For address translation in such a packet including an IP address in the payload portion, translation in both the IP header and the payload can be performed simultaneously by a protocol translation means with support for an application level gateway (ALG), such as SOCKS64. However, because all translation process is performed by application software, high-speed protocol translation is hard to execute. 
     Meanwhile, protocol translation by NAT-PT involves only IP header change and the translation function is performed by dedicated hardware, and, therefore, high-speed translation can be executed. For ordinary inter-protocol communication, in most cases, translation processing is required only in the IP header and translation is not necessary in the payload. In view hereof, the NAT-PT is advantageous in terms of a total processing speed. However, if translation is necessary in the payload, an ALG must be provided separately and the packet must be transferred to the ALG for further translation after translation in the IP header is completed by an IPv4/IPv6 translator. 
     When, in addition to the NAT-PT, one or more ALGs for translation in the payload, such as DNS proxy servers, are used, it is a problem where address translation rules are maintained. In view of processing speed, it is considered the best that an IPv4/IPv6 translator which most frequently refers to the address translation rules maintains these rules. In such cases, the one or more ALGs query the IPv4/IPv6 translator about a substitutive address when executing address translation in the payload. 
     [Non-Patent Document Cited 1] 
     E. Nordmark, “RFC2765,” “online,” February, 2000, Internet &lt;URL: http://www.ietf.org/rfc/rfc2765.txt&gt; 
     [Non-Patent Document Cited 2] 
     H. Kitamura, “RFC3089,” “online,” April, 2001, Internet &lt;URL: http://www.ietf.org/rfc/rfc3089.txt&gt; 
     [Non-Patent Document Cited 3] 
     J. Hagino, et al., “RFC3142,” “online,” July, 2001, Internet &lt;URL: http://www.ietf.org/rfc/rfc3142.txt&gt; 
     [Japanese Patent Document Cited 1] 
     JP-A No. 156710/2000 
     If an IPv4 network and an IPv6 network which are of a large scale are interconnected, parallel use of a plurality of IPv4/IPv6 translators at one connection point is conceivable for load balancing and fail-free operation. In network topology variety, IPv4 private networks using private address allocation defined in RFC1918 are supposed to connect to an IPv6 network, where datagram is transferred from the IPv4 networks to the IPv6 network and vice versa and datagram transferred between different IPv4 networks, taking advantage of combination of IPv4 to IPv6 translation and IPv6 to IPv4 translation. 
     However, conventional ALGs to make translation in the payload were unable to determine which one of multiple IPv4/IPv6 translators to which to send a request. 
     SUMMARY OF THE INVENTION 
     The present invention, which has been made in view of the above-described problem of prior art, provides a network topology with a packet-switched communication apparatus (DNS proxy server) including means for retaining mappings between IPv6 address prefix values, each denoting a network, and IPv4/IPv6 translators, each of which uses a specific prefix. The packet-switched communication apparatus is configured to, when translation of IP address data in the payload portion of an IP packet of name resolution request is required, refer to an address included in the packet, select an IPv4/IPv6 translator mapped to the prefix value of the address, and send a request for a substitutive IP address to the IPv4/IPv6 translator. 
     To make translation in the payload portion of a reverse DNS lookup packet, the communication apparatus judges either a reverse lookup on an IPv4 address or a reverse lookup on an IPv6 address, contained in the packet before the translation, judges either a query or a response packet, refers to an IPv6 address located in a different portion of the packet, according to the judgment, and selects an IPv4/IPv6 translator. 
     The communication apparatus of the present invention may map a plurality of IPv4/IPv6 translators to one IPv6 address prefix. 
     According to the packet-switched communication apparatus (DNS proxy server) of the present invention, in a network system where IPv4 networks and an IPv6 network are interconnected via a plurality of IPv4/IPv6 translators, it is possible to obtain a substitutive address for translation in the payload from an appropriate IPv4/IPv6 translator and to make IP address translation to a higher-layer IP address and vice versa. It is also possible to map a plurality of IPv4/IPv6 translators to one IPv6 address prefix. Each time a request for address data is to be sent to one of the IPv4/IPv6 translators mapped to the prefix, the IPv4/IPv6 translator to which the request is sent can be switched from one to another so that load balancing across the IPv4/IPv6 translators becomes feasible. 
     In the network system of the present invention, a reverse DNS lookup can be performed to look up of an IPv4 address from the IPv6 network side and to look up of an IPv6 address from the IPv4 network side. Even in a mixed IPv4 network and IPv6 network topology, services such as e-mail and client-server system can be implemented as is the case in a conventional topology consisting entirely of IPv4 networks. 
     If a substitutive address cannot be obtained from an IPv4/IPv6 translator to which a request for such address was sent, the request for the substitutive address is re-transmitted to another IPv4/IPv6 translator mapped to the same prefix. Accordingly, trouble due to a translator fault or the like can be avoided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram depicting the topology of a communications network system including a DNS proxy server in accordance with A first embodiment of the present invention; 
         FIG. 2  is a block diagram showing the configuration of the DNS proxy server of the first embodiment; 
         FIG. 3  shows exemplary contents of a prefix-translator reference table relevant to the first embodiment; 
         FIG. 4  shows exemplary contents of a message ID-IPv6 address reference table relevant to the first embodiment; 
         FIG. 5  shows the format of a DNS message; 
         FIG. 6  shows the format of a header section of the DNS message; 
         FIG. 7  shows the format of a question section of the DNS message; 
         FIG. 8  shows the format common for answer, authority, and additional sections of the DNS message; 
         FIG. 9  is a sequence diagram of a process for completing a request issued from an IPv4 client for a reverse lookup on the address of an IPv6 client in accordance with the first embodiment; 
         FIG. 10  is a sequence diagram of a process for completing a request issued from an IPv6 client for a reverse lookup on the address of an IPv4 client in accordance with the first embodiment; 
         FIG. 11  is a diagram depicting the topology of a communications network system including a DNS proxy server in accordance with a second embodiment of the present invention; 
         FIG. 12  is a block diagram showing the configuration of the DNS proxy server of the second embodiment; 
         FIG. 13  shows exemplary contents of a prefix-network ID reference table relevant to the second embodiment; 
         FIG. 14  shows exemplary contents of a network ID-translator reference table; 
         FIG. 15  is a diagram depicting the topology of a communications network system including a node in which a DNS proxy server and IPv4/IPv6 translators are integrated, according to a third embodiment of the present invention; and 
         FIG. 16  is a block diagram showing the configuration of an IPv4/IPv6 gateway of the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. 
     A first embodiment of the present invention is first discussed. 
       FIG. 1  is a diagram depicting the topology of a communications network system including a DNS proxy server in accordance with the first embodiment of the present invention. 
     An IPv6 network  600  is connected to an IPv4 network  400  via an IPv4/IPv6 translator  101  and also connected to an IPv4 network  410  via an IPv4/IPv6 translator  102 . 
     The IPv6 network  600  uses only the IPv6 communication protocol and a DNS proxy server  1   a , a DNS server  601 , and an IPv6 client  602  are connected thereto. The DNS proxy server  1   a  receives a name resolution request on behalf of the DNS server  601 , determines which IPv4/IPv6 translator serves a device whose IP address is sought by the name resolution request (that is, to find that the device is attached to which network connected to which IPv4/IPv6 translator), and translates the FQDN specified in the payload of the name resolution request packet. The DNS server  601  has a table of address/name mapping for devices connected to the IPv6 network  600 . The IPv6 client  602  is a computer as a data communication terminal. 
     The IPv4 network  400  uses only the IPv4 communication protocol and a DNS server  401  and an IPv4 client  402  are connected thereto. The DNS server  401  has a table of address/name mapping for devices connected to the IPv4 network  400 . The IPv4 client  402  is a computer as a data communication terminal. 
     The IPv4 network  410  also uses only the IPv4 communication protocol and a DNS server  411  and an IPv4 client  412  are connected thereto. The DNS server  411  has a table of address/name mapping for devices connected to the IPv4 network  410 . The IPv4 client  412  is a computer as a data communication terminal. 
     Because the IPv6 network  600  uses only the IPv6 communication protocol, it is only possible to specify an IPv6 address in a reverse lookup query. On the other hand, because both the IPv4 network  400  and the IPv4 network  410  use only the IPv4 communication protocol, it is only possible to specify an IPv4 address in a reverse lookup query. 
     The IPv4/IPv6 translators  101  and  102  are devices for translating IPv4 to IPv6 addressing and vice versa for the destination and source addresses of an IP packet transferred thereto across the network to which each is connected. 
       FIG. 2  is a block diagram showing the configuration of the DNS proxy server  1   a  according to the first embodiment. 
     The DNS proxy server  1   a  is comprised of an I/O interface  11 , a DNS message analyzer  12 , a substitutive address query message generator  13 , a substitutive address notification message analyzer  14 , a DNS message translator  15 , an IPv4/IPv6 translator information manager  16 , and a message ID manager  18 . These components are embodied in electronic devices, for example, CPU, RAM, etc. The I/O interface  11  transmits and receives an IP packet to be passed across the IPv4 networks and the IPv6 network. 
     The DNS message analyzer  12  analyzes a received message, determines the type of the message such as a reverse lookup query, a response, etc., and extracts address information, for example, an IPv4 or IPv6 address specified in a reverse lookup query, according to the type of the message. 
     The substitutive address query message generator  13  generates a query message to an IPv4/IPv6 translator about an address for IPv4/IPv6 translation from the address information extracted by the DNS message analyzer  12 . 
     The substitutive address notification message analyzer  14  receives a response message to a message generated by the substitutive address query message generator  13  and analyzes the message content. 
     The DNS message translator  15  replaces an IPv4 address or IPv6 address specified in a reverse lookup query or response message by a substitutive address notified from an IPv4/IPv6 address translator. 
     The IPv4/IPv6 translator information manager  16  holds a prefix-translator reference table  160  in which mappings between IPv6 address prefix values and IPv4/IPv6 translators are defined. 
     The message ID manager  18  holds a message ID-IPv6 address reference table  190  in which mappings between IPv4 message IDs and source IPv6 addresses are defined. 
       FIG. 3  shows exemplary contents of the prefix-translator reference table  160  held by the IPv4/IPv6 translator information manager  16 . 
     This prefix-translator reference table  160  consists of a column  161  for prefix entry and a column  162  for IPv4/IPv6 translator information entry. The prefix entry column  161  consists of a field  163  in which a 128-bit IPv6 address value is registered and a field  164  in which the length in bits of the prefix part of the IPv6 address, from the beginning, is registered. The IPv4/IPv6 translator information column  162  consists of a field  165  in which the IPv6 address of an IPv4/IPv6 translator is registered and a field  166  in which the User Datagram Protocol (UDP) port number of the IPv4/IPv6 translator is registered. 
       FIG. 4  shows exemplary contents of the message ID-IPv6 address reference table  190  held by the message ID manager  18 . 
     This message ID-IPv6 address reference table  190  consists of a column  191  for IPv6 address entry and a column  192  for message ID entry. 
     Then, a reverse lookup through the DNS in the communications network system including the DNS proxy server of the first embodiment is discussed. 
     The reverse lookup through the DNS is implemented as follows. An IP address on which a reverse lookup is to be performed is converted to a name (FQDN) for reverse lookup and a query is made to a DNS server about the name (PTR (PoinTeR) record) corresponding to the FQDN. 
     If a reverse lookup on an IPv4 address is performed, an FQDN appended with “in-addr.arpa” is used for reverse lookup. This FQDN for reverse lookup is obtained by dividing a 32-bit IPv4 address into four 8-bit portions, converting all the 8-bit values into decimal values, reversing the order of the four decimal values, wherein each value is separated with a dot, and appending “in-addr.arpa” to the number part. If, for example, IPv4 address “192.168.1.142” is resolved to a domain name, an FQDN for reverse lookup “142.1.168.192.in-addr.arpa” is generated. 
     If a reverse lookup on an IPv6 address is performed, an FQDN appended with “ip6.arpa” is used for reverse lookup. This FQDN for reverse lookup is obtained by converting a 128-bit IPv6 address into hexadecimal values, reversing the order of the hexadecimal values, wherein each value is separated with a dot, and appending “ip6.arpa” to the number part. For example, IPv6 address “2001:1001::200:e2ff:fe53:986b” is resolved to an FQDN for reverse lookup “b.6.8.9.3.5.e.f.f.f.2.e.0.0.2.0.0.0.0.0.0.0.0.0.1.0.0.1 .1.0.0.2.ip6.arpa”. 
     By general convention, the FQDN for reverse lookup of IPv6 is formulated as “¥[x20011001000000000200e2fffe53986b/128¥].ip6.arpa” (where “¥” is often used in Japanese conventional typing, instead of a backslash symbol which would be applied more commonly in other countries). 
     In some system implementation, an IPv6 address may be resolved to an FQDN appended with “in6.int” instead of “ip6.arpa” for reverse lookup. 
     Next, details on the above-mentioned DNS message are discussed. 
     As is shown in  FIG. 5 , a DNS message is composed of a header section  21 , a question section  22 , an answer section  23 , an authority section  24 , and an additional section  25 . The sections other than the header section  21  may not always exist as a complete set. 
     As is shown in  FIG. 6 , the header section  21  is made up of an ID field  210  to contain a message ID uniquely assigned to the message, a QR field  211  to contain a flag to indicate if the message is a response to a query, an OPCODE field  212  to contain a value to designate the query type, a miscellaneous flags filed  213 , a reserved field  214 , an RCODE field  215  to contain a value to indicate the response type if the message is a response, a QDCOUNT field  216  to contain a value to indicate the number of entries contained in the question section, an ANCOUNT field  217  to contain a value to indicate the number of resource records contained in the answer section  23 , an NSCOUNT field  218  to contain a value to indicate the number of resource records contained in the authority section  24 , and an ARCOUNT field  219  to contain a value to indicate the number of resource records contained in the additional section  25 . 
     As is shown in  FIG. 7 , the question section  22  is made up of a QNAME field  220 , a QTYPE field  221 , and a QCLASS field  222 . The QNAME field  220  is a field to contain an FQDN for query and this field contains an FQDN for reverse lookup if the message is a reverse lookup query. The QTYPE field  221  contains a value to indicate the type of resource record for which the query is made. 
     The answer section  23 , authority section  24 , and additional section  25  are made up of fields which are shown in  FIG. 8  as those common to these sections. The fields comprise a NAME field  230  to contain an FQDN for query, a TYPE field  231  to contain a value to indicate the type of resource record for which the query is made, a TTL field to contain a time to live during which the query response is retained on the cache of the DNS proxy server  1   a , an RDATA field  232  to contain a resolved-to-name (PTR record) as the query response, and other fields. 
     For data length compression purposes, if all or a part of an FQDN string is duplicated across two or more fields, one field should contain the complete FQDN string and other fields should contain a pointer to the beginning part of the FQDN string contained in the one field, instead of containing the FQDN. Upon receiving this DNS message, a device can obtain the FQDN by reading the pointer and referring to the field content pointed by the pointer. 
     If the DNS message is a reverse lookup query, the QR field  211  in the header section contains a flag value of 0 and the header section is followed by only the fields of the question section  22 . 
     If the DNS message is a response to a reverse lookup query, the ID field  210  in the header section  21  contains a copy of the ID of the associated query message and the QR field  211  contains a flag value of 1. In the question section  22  of the response message, the contents of the question section  22  of the query message are stored as is. 
     In the reserve lookup query message of DNS, furthermore, the NAME field  230  in the answer section contains the above-mentioned IPv4 or IPv6 FQDN for reverse lookup, the TYPE field  231  contains a value of 12 indicating a PTR record, and the RDATA field  232  contains an FQDN assigned to the device having the specified address. However, because the content of the NAME field  230  in the answer section is usually the same as the content of the corresponding field in the question section, the NAME field  230  contains a pointer to the beginning of the FQDN for reverse lookup contained in the QNAME field  220  in the question section. 
     Then, how the DNS proxy server  1   a  operates upon receiving a DNS message is discussed. 
     DNS proxy server operation in a first instance where the DNS proxy server  1   a  receives a query of a reverse lookup on an IPv6 address is first discussed. 
     A DNS message received through the I/O interface  11  is passed to the DNS message analyzer  12 . 
     The DNS message analyzer  12  analyzes the received DNS message content. If the DNS message includes an FQDN for reverse lookup appended with “ip6.arpa” (or “ip6.int.”), the DNS message analyzer  12  determines that the DNS message is a reverse lookup query for a PTR record and extracts an IPv6 address from the FQDN for reverse lookup in this reverse lookup DNS message. Then, the DNS message analyzer  12  passes the IPv6 address to the substitutive address query message generator  13 . 
     The substitutive address query message generator  13  searches the prefix-translator reference table  160  held on the IPv4/IPv6 translator information manager  16 , using the extracted IPv6 address, obtains IPv4/IPv6 translator information mapped to the prefix of the previously extracted IPv6 address, and selects one IPv4/IPv6 translator. At this time, if a plurality of IPv4/IPv6 translators are obtained by the search, the message generator obtains information about the plurality of translators and selects one (or a plurality) of the IPv4/IPv6 translators. 
     Then, a query for an IPv4 address corresponding to the extracted IPv6 address is sent to the selected IPv4/IPv6 translator. At this time, if the corresponding IPv4 address cannot be obtained from the IPv4/IPv6 translator to which the query was sent, the same query for an IPv4 address corresponding to the extracted IPv6 address is sent to another selected IPv4/IPv6 translator. The query is repeated until the IPv4 address is obtained or sending the query to all the IPv4/IPv6 translators registered in the above prefix-translator reference table is completed. In this relation, a maximum number of times of query repetition may be set at a given number that an administrator should set beforehand. 
     A response including the IPv4 address corresponding to the IPv6 address sent back from the IPv4/IPv6 translator is received through the I/O interface  11  and received by the substitutive address notification message analyzer  14 . The substitutive address notification message analyzer  14  extracts the IPv4 address from the response message and, thereby, the IPv4 address corresponding to the IPv6 address is obtained. The thus obtained IPv4 address is passed to the DNS message translator  15 . 
     The DNS message translator  15  translates the obtained IPv4 address into an FQDN for reverse lookup and replaces the FQDN for IPv6 reverse lookup included in the question section in the initially received reverse lookup DNS message by the FQDN for reverse lookup including the IPv4 address, thus generating a new reverse lookup DNS message. The thus generated new reverse lookup DNS message is transmitted through the I/O interface  11  to another DNS server that holds name resolution information. 
     Otherwise, if such an IPv4 address cannot be obtained eventually even after repetitive queries to all the IPv4/IPv6 translator (after a predetermined number of times the query has been repeated), the received reverse lookup DNS message is transmitted as is to another DNS server that holds name resolution information. 
     Then, DNS proxy server operation in a second instance where the DNS proxy server  1   a  receives a query of a reverse lookup on an IPv4 address is discussed. 
     A DNS message received through the I/O interface  11  is passed to the DNS message analyzer  12 . 
     The DNS message analyzer  12  analyzes the received DNS message content. If the DNS message includes an FQDN for reverse lookup appended with “in-addr.arpa”, the DNS message analyzer  12  determines that the DNS message is a reverse lookup query for a PTR record and extracts an IPv4 address from the FQDN for reverse lookup in this reverse lookup DNS message. Then, the DNS message analyzer  12  passes the IPv4 address to the substitutive address query message generator  13 . 
     The substitutive address query message generator  13  searches the prefix-translator reference table  160  held on the IPv4/IPv6 translator information manager  16 , using the source IPv6 address of the reverse lookup query, obtains IPv4/IPv6 translator information mapped to the prefix of the source IPv6 address, and selects one (or a plurality) of the IPv4/IPv6 translators. A query for an IPv6 address corresponding to the IPv4 address is sent to the selected IPv4/IPv6 translator. At this time, as noted above, if a plurality of IPv4/IPv6 translators are obtained, the query may be repeated to the IPv4/IPv6 translators. 
     A response including the IPv6 address corresponding to the IPv4 address sent back from the IPv4/IPv6 translator as the result of the query is received through the I/O interface  11  and received by the substitutive address notification message analyzer  14 . The substitutive address notification message analyzer  14  extracts the IPv6 address from the response message and, thereby, the IPv6 address corresponding to the IPv4 address is obtained. The thus obtained IPv6 address is passed to the DNS message translator  15 . 
     The DNS message translator  15  translates the obtained IPv6 address into an FQDN for reverse lookup and replaces the IPv4 FQDN for reverse lookup included in the question section in the initially received reverse lookup DNS message by the FQDN for reverse lookup including the IPv6 address, thus generating a new reverse lookup DNS message. The thus generated new reverse lookup DNS message is transmitted through the I/O interface  11  to another DNS server that holds name resolution information. 
     At this time, the message ID included in the reverse lookup DNS message that is transmitted to another DNS server is mapped to the source IPv6 address of the reverse lookup query before the FQDN replacement and this mapping is registered into the message ID-IPv6 address reference table  190  (see  FIG. 4 ) on the message ID manager  18 . 
     Otherwise, if such an IPv6 address cannot be obtained eventually, the received reverse lookup DNS message is transmitted as is to another DNS server that holds name resolution information. 
     Next, DNS proxy server operation in a third instance where the DNS proxy server  1   a  receives a response to the query of a reverse lookup on an IPv4 address is discussed. 
     A DNS message received through the I/O interface  11  is passed to the DNS message analyzer  12 . 
     The DNS message analyzer  12  analyzes the received DNS message content. When the DNS message analyzer  12  determines that the DNS message is a reply to a reverse lookup DNS message (the result of name resolution), it extracts an IPv4 address from the FQDN for reverse lookup stored in the question section  22  (see  FIG. 6 ) of the DNS message, and passes the IPv4 address to the substitutive address query message generator  13 . 
     The substitutive address query message generator  13  searches the prefix-translator reference table  160  held on the translator information manager  16 , using the source IPv6 address of the reply to the reverse lookup, obtains IPv4/IPv6 translator information mapped to the prefix of the IPv6 address of the source that transmitted the reply to the reverse lookup message, and selects one (or a plurality) of the IPv4/IPv6 translators. 
     A query for an IPv6 address corresponding to the IPv4 address extracted by the DNS message analyzer  12  is transmitted through the I/O interface  11  to the selected IPv4/IPv6 translator, thus issuing a request for an IPv6 address to replace the extracted IPv4 address. At this time, as noted above, if information about a plurality of IPv4/IPv6 translators is obtained and the plurality of IPv4/IPv6 translators are selected, the query may be repeated to the IPv4/IPv6 translators. 
     When a response message including the IPv6 address corresponding to the IPv4 address is sent back from the IPv4/IPv6 translator, the response message is received through the I/O interface  11  and received by the substitutive address notification message analyzer  14 . The substitutive address notification message analyzer  14  extracts the IPv6 address from the response message and passes the IPv6 address to the DNS message translator  15 . 
     The DNS message translator  15  replaces the IPv4 FQDN for reverse lookup stored in the question section  22  and the answer section  23  (see  FIG. 7 ) of the reverse lookup response message received from a DNS server by an FQDN for reverse lookup converted from the IPv6 address received from the IPv4/IPv6 translator, thus generating a new response message, and transmits the new response message to the query sender of the reverse lookup DNS message through the I/O interface  11 . 
     At this time, if the pointer to the QNAME field in the question section  22  is stored in the answer section  23 , the pointer position is changed by the above FQDN replacement. Accordingly, the pointer must be corrected to a pointer to the beginning of the replaced FQDN string for reverse lookup in the question section  22 . 
     Otherwise, if the IPv6 address cannot be obtained eventually, the received response message is transmitted as is to the query sender of the reverse lookup DNS message. 
     Finally, DNS proxy server operation in a fourth instance where the DNS proxy server  1   a  receives a response to the query of a reverse lookup on an IPv6 address is discussed. 
     A DNS message received through the I/O interface  11  is passed to the DNS message analyzer  12 . 
     The DNS message analyzer  12  analyzes the received DNS message content. When the DNS message analyzer  12  determines that the DNS message is a response to a query for a PTR record associated with an “ip6.arpa” or “ip6.int” domain in a DNS message (the result of name resolution), it extracts an IPv6 address from the FQDN for reverse lookup stored in the question section  22  (see  FIG. 6 ) of the DNS message, and passes the IPv6 address to the substitutive address query message generator  13 . 
     The substitutive address query message generator  13  searches message ID-IPv6 address reference table  190  held on the message ID manager  18 , using the message ID of the received response to the reverse lookup query, obtains the IPv6 address mapped to the message ID. 
     Furthermore, the substitutive address query message generator  13  searches prefix-translator reference table  160  held on the IPv4/IPv6 translator information manager, using the obtained IPv6 address, obtains IPv4/IPv6 translator information mapped to the obtained IPv6 address prefix, and selects one (or a plurality) of the IPv4/IPv6 translators. 
     A query for an IPv4 address corresponding to the IPv6 address extracted by the DNS message analyzer  12  is transmitted through the I/O interface  11  to the selected IPv4/IPv6 translator, thus issuing a request for an IPv4 address to replace the extracted IPv6 address. At this time, as noted above, if a plurality of IPv4/IPv6 translators are obtained, the query may be repeated to the IPv4/IPv6 translators. 
     When a response message including the IPv4 address corresponding to the IPv6 address is sent back from the IPv4/IPv6 translator, the response message is received through the I/O interface  11  and received by the substitutive address notification message analyzer  14 . The substitutive address notification message analyzer  14  extracts the IPv4 address from the response message and passes the IPv6 address to the DNS message translator  15 . The DNS message translator  15  replaces the IPv6 FQDN for reverse lookup stored in the question section  22  and the answer section  23  (see  FIG. 7 ) of the reverse lookup response message received from a DNS server by an FQDN for reverse lookup converted from the IPv4 address received from the IPv4/IPv6 translator, thus generating a new response message, and transmits the new response message to the query sender of the reverse lookup DNS message through the I/O interface  11 . 
     At this time, if the pointer is stored in the answer section  23 , the pointer position is changed by the above FQDN replacement. Accordingly, the pointer must be corrected to a pointer to the beginning of the replaced FQDN string for reverse lookup in the question section  22 . 
     Otherwise, if the IPv4 address cannot be obtained eventually, the received response message is transmitted as is to the query sender of the reverse lookup DNS message. 
     Then, in the communications network system of the first embodiment, when an IPv4 client issues a request for a reverse lookup on the address of an IPv6 client, how the DNS proxy server and related devices cooperate to complete the request is discussed. 
     In the topology shown in  FIG. 1 , it is assumed that the addresses and names of the clients are initially set as below. 
     The IPv4 client  412  is assigned FQDN “hostX.v4.net” and IPv4 address “192.168.0.2”. 
     The IPv6 client  602  is assigned FQDN “hostY.v6.net” and IPv6 address “2001::1”. The DNS server  601  connected to the IPv6 network  600  is configured to be able to reply to a request for a reverse lookup on an IPv6 address that falls within a subnet “2001::/96” and to make a name resolution in this subnet range. Moreover, the DNS proxy server  1   a  is set up to transfer a reverse lookup query for an IPv6 address that falls within the subnet “2001::/96” to the DNS server  601 . 
     The IPv4/IPv6 translator  101  assigns a temporary IPv4 address “10.0.0.1” to the IPv6 client  602  and maps the IPv6 address “2001::1” to the IPv4 address “10.0.0.1”; this mapping is registered in the address translation table. 
     The IPv4/IPv6 translator  101  generates, for all devices connected to the IPv4 network  400 , a temporary IPv6 address by adding a prefix “3ffe:1::/96” to the device&#39;s IPv4 address. Moreover, address translation information on the DNS proxy server  1   a  is registered on the IPv4/IPv6 translator  101  as well beforehand. 
     The IPv4/IPv6 translator  102  generates, for all devices connected to the IPv4 network  410 , a temporary IPv6 address by adding a prefix “3ffe:2::/96” to the device&#39;s IPv4 address. Address translation information on the DNS proxy server  1   a  is registered on the IPv4/IPv6 translator  102  as well beforehand as is the case for the IPv6 translator  101 . 
     The DNS proxy server  1   a  holds the mapping between the IPv6 address prefix “3ffe:1::/96” and the IPv4/IPv6 translator  101  and the mapping between the prefix “3ffe:2::/96” and the IPv4/IPv6 translator  102 . 
       FIG. 9  is a sequence diagram of a process for completing a request issued from the IPv4 client  402  for a reverse lookup on the address of the IPv6 client  602 . 
     The IPv4 client  402  transmits a message A of query for the name of the device assigned the IPv4 address “10.0.0.1” to the nearest DNS server  401 . The DNS server  401  transfers the message A to the DNS proxy server  1   a , if the server is unable to make the name resolution by itself. In the course of the message transfer from the IPv4 network  400  to IPv6 network  600 , this message A is converted from an IPv4 packet to IPv6 packet form at the IPv4/IPv6 translator  101  and the IPv6 packet message arrives on the DNS proxy server  1   a.    
     Upon receiving the message A, the DNS proxy server  1   a  parses an FQDN “1.0.0.10.in-addr.arpa” for reverse lookup stored in the question section  22  of the message and extracts the IPv4 address “10.0.0.1”. 
     Next, the DNS proxy server  1   a  searches the prefix-translator reference table  160  on the IPv4/IPv6 translator information manager  16 , using the source IPv6 address of the message A, obtains information of an IPv4/IPv6 translator  101  mapped to the prefix “3ffe:1::/96” of the source address of the message A, and selects the IPv4/IPv6 translator  101  mapped to the prefix. 
     Then, the DNS proxy server  1   a  transmits an IP packet E of query for a substitutive IPv6 address corresponding to the address “10.0.0.1” to the selected IPv4/IPv6 translator  101 . 
     Upon having received the IP packet E (query for a substitutive IPv6 address), the IPv4/IPv6 translator  101  refers to the address translation table to search for an IPv6 address corresponding to the address “10.0.0.1”, obtains “2001::1” as the corresponding IPv6 address, and transmits a reply IP packet F for notification of the substitutive IPv6 address back to the DNS proxy server  1   a.    
     The DNS proxy server  1   a  generates an IPv6 FQDN for reverse lookup from the IPv6 address included in the IP packet F (the substitutive IPv6 address as the reply) received from the IPv4/IPv6 translator  101 , replaces the FQDN “1.0.0.10.in-addr.arpa” for reverse lookup in the question section  22  of the message A by the generated FQDN “¥[x20010000000000000000000000000001/128¥].ip6.arpa” for reverse lookup, and transmits a message A′ in which the QNAME field in the question section contains the converted-to-FQDN to a DNS server  601 . At this time, the DNS proxy server  1   a  maps the message ID of the message A′ to the source IPv6 address of the message A and stores this mapping into the message ID-IPv6 address reference table  190  on the message ID manager. 
     Upon having received the message A′, in response to the message A′, the DNS server  601  transmits a reply message B for notification of a PTR record (domain name) “hostY.v6.net” corresponding to the FQDN “¥[x20010000000000000000000000000001/128¥].ip6.arpa” back to the DNS proxy server  1   a.    
     In the message B, the QNAME field in the question section  22  contains the IPv6 FQDN “¥[x20010000000000000000000000000001/128¥].ip6.arpa” for reverse lookup and the NAME field in the answer section  23  contains the pointer to the beginning of the FQDN string in the QNAME field in the question section  22 . 
     Upon receiving the message B, the DNS proxy server  1   a  parses the FQDN “¥[x20010000000000000000000000000001/128¥].ip6.arpa” for reverse lookup stored in the question section  22  of the message B and extracts the IPv6 address “2001::1” of the IPv6 client  602 . 
     Then, the DNS proxy server  1   a  searches the message ID-IPv6 address reference table  190  on the message ID manager  18 , using the ID of the message B, and obtains the source address (IPv6 address) “3ffe:1::c0a8:1” mapped to the ID of the message B, because the ID of the message B is the same as the previously registered ID of the message A′. Furthermore, the DNS proxy server  1   a  searches the prefix-translator reference table  160  on the IPv4/IPv6 translator information manager  16 , using the thus obtained IPv6 address, obtains information of the IPv4/IPv6 translator  101  mapped to the IPv6 prefix “3ffe:1::/96”, and selects the IPv4/IPv6 translator  101  mapped to the prefix. 
     Then, the DNS proxy server  1   a  transmits an IP packet G of query for a substitutive IPv4 address corresponding to the address “2001::1” to the IPv4/IPv6 translator  101 . 
     Upon having received the IP packet G (query for a substitutive IPv4 address), the IPv4/IPv6 translator  101  refers to the address translation table to search for an IPv4 address corresponding to the address “2001::1”, obtains “10.0.0.1” as the corresponding IPv4 address, and transmits a reply IP packet H for notification of the substitutive IP address back to the DNS proxy server  1   a.    
     The DNS proxy server  1   a  generates an IPv4 FQDN for reverse lookup from the IPv4 address included in the IP packet H (the substitutive IPv4 address as the reply) received from the IPv4/IPv6 translator  101  and replaces the FQDN “¥[x20010000000000000000000000000001/128¥].ip6.arpa” for reverse lookup in the question section of the message B by the generated FQDN “1.0.0.10.in-addr.arpa” for reverse lookup. The DNS proxy server  1   a  transmits a message B′ in which the QNAME field in the question section contains the converted-to-FQDN and the pointer to the QNAME field has been modified to the IPv4 client  402 . 
     In the course of the message transfer from the IPv6 network  600  to the IPv4 network  400 , the message B′ is converted from an IPv4 packet to IPv6 packet form at the IPv4/IPv6 translator  101  and the IPv6 packet message arrives on the IPv4 client  402 . 
     As the result of receiving the message B′, the IPv4 client  402  obtains information that the name of the device having the IPv4 address “10.0.0.1” is “hostY.v6.net”. 
     Through the above-described process, the reverse lookup on the IPv6 address from the IPv4 network side is complete. 
     Next, when an IPv6 client issues a request for a reverse lookup on the address of an IPv4 client, how the DNS proxy server and related devices cooperate to complete the request is discussed. 
       FIG. 10  is a sequence diagram of a process for completing a request issued from the IPv6 client  602  for a reverse lookup on the address of the IPv6 client  412 . 
     The IPv6 client  602  transmits a message C of query for the name of the device assigned the IPv6 address “3ffe:2::c0a8:2” to the nearest DNS server  601 . The DNS server  601  transfers the message C to the DNS proxy server  1   a , if the server is unable to make the name resolution by itself. 
     Upon receiving the message C, the DNS proxy server  1   a  parses an FQDN “¥[x3ffe00020000000000000000c0a80002/128¥].ip6.arpa” for reverse lookup stored in the question section  22  of the message and extracts the IPv6 address “3ffe:2::c0a8:2”. 
     Next, the DNS proxy server  1   a  searches the prefix-translator reference table  160  on the IPv4/IPv6 translator information manager  16 , using the extracted IPv6 address, obtains information of an IPv4/IPv6 translator  102  mapped to the prefix “3ffe:2::/96” of the IPv6 address of the device about which the query is made by using the message C, and selects the IPv4/IPv6 translator  102  mapped to the prefix. 
     Then, the DNS proxy server  1   a  transmits an IP packet I of query for a substitutive IPv4 address corresponding to the address “3ffe:2::c0a8:2” to the selected IPv4/IPv6 translator  102 . 
     Upon having received the IP packet I (query for a substitutive IPv4 address), the IPv4/IPv6 translator  102  refers to the address translation table to search for an IPv4 address corresponding to the address “3ffe:2::c0a8:2”, obtains “192.168.0.2” as the corresponding IPv4 address, and transmits a reply IP packet J for notification of the substitutive IP address back to the DNS proxy server  1   a.    
     The DNS proxy server  1   a  generates an IPv4 FQDN for reverse lookup from the IPv4 address included in the IP packet J (the substitutive IPv4 address as the reply) received from the IPv4/IPv6 translator  102 , replaces the FQDN “¥[x3ffe00020000000000000000c0a80002/128¥].ip6.arpa” for reverse lookup in the question section  22  of the message C by the generated FQDN “2.0.168.192.in-addr.arpa” for reverse lookup, and transmits a message C′ in which the QNAME field in the question section contains the converted-to-FQDN to a DNS server  411 . In the course of the message transfer from the IPv6 network  600  to the IPv4 network  400 , the message C′ is converted from an IPv6 packet to IPv4 packet form at the IPv4/IPv6 translator  102  and the IPv4 packet message arrives on the DNS server  411 . 
     Upon having received the message C′, in response to the message C′, the DNS server  411  transmits a reply message D for notification of a PTR record (domain name) “hostX.v4.net” corresponding to the FQDN “2.0.168.192.in-addr.arpa” back to the DNS proxy server  1   a.    
     In the course of the message transfer from the IPv4 network  400  to the IPv6 network  600 , the message D is converted from an IPv4 packet to IPv6 packet form at the IPv4/IPv6 translator  102  and the IPv6 packet message arrives on the DNS proxy server  1   a . In this message D, the QNAME field in the question section  22  contains the IPv4 FQDN “2.0.168.192.in-addr.arpa” for reverse lookup and the NAME field in the answer section  23  contains the pointer to the beginning of the FQDN string in the QNAME field in the question section  22 . 
     Upon receiving the message D, the DNS proxy server  1   a  parses the FQDN “2.0.168.192.in-addr.arpa” for reverse lookup stored in the question section  22  of the message D and extracts the IPv4 address “192.168.0.2” of the IPv4 client  412 . 
     Next, the DNS proxy server  1   a  searches the prefix-translator reference table  160  on the IPv4/IPv6 translator information manager  1 , using the source IPv6 address of the message D, obtains information of an IPv4/IPv6 translator  102  mapped to the prefix “3ffe:2::/96” of the source address of the message D, and selects the IPv4/IPv6 translator  102  mapped to the prefix. 
     Then, the DNS proxy server  1   a  transmits an IP packet K of query for a substitutive IPv6 address corresponding to the “192.168.0.2” address (query for a substitutive IPv6 address) to the IPv4/IPv6 translator  102 . 
     The IPv4/IPv6 translator  102  refers to the address translation table to search for an IPv6 address corresponding to the “192.168.0.2” address, obtains “3ffe:2::c0a8:2” as the corresponding IPv6 address, and transmits a reply IP packet L for notification of the substitutive IP address back to the DNS proxy server  1   a.    
     The DNS proxy server  1   a  generates an IPv6 FQDN for reverse lookup from the IPv6 address included in the IP packet L (the substitutive IPv6 address as the reply) received from the IPv4/IPv6 translator  102  and replaces the FQDN “2.0.168.192.in-addr.arpa” for reverse lookup in the question section of the message D by the generated FQDN “¥[x3ffe00020000000000000000c0a80002/128¥].ip6.arpa”. The DNS proxy server  1   a  transmits a message D′ in which the QNAME field in the question section contains the converted-to-FQDN and the pointer to the QNAME field has been modified to the IPv6 client  602 . 
     As the result of receiving the message D′ with the replaced FQDN, the IPv6 client  602   a  obtains information that the name of the device having the IPv6 address “3ffe:2::c0a8:2” is “hostX.v4.net”. 
     Through the above-described process, the reverse DNS lookup on the IPv4 address from the IPv6 network side is complete. 
     In the first embodiment discussed hereinbefore, even if an IPv4 network and an IPv6 network are connected via a plurality of IPv4/IPv6 translators, the IPv6 network includes the DNS proxy server  1   a  for serving a request to a DNS server and this DNS proxy server  1   a  reads the prefix of a destination address of a DNS request message, selects an IPv4/IPv6 translator mapped to the prefix, requests the selected IPv4/IPv6 translator to make IPv4/IPv6 address translation, and sends a request modified with a translated-to-address to a DNS server. Thereby, a reverse lookup on an IP address requested can be processed through a DNS server even in the network topology via a plurality of IPv4/IPv6 translators. 
     Next, a second embodiment of the invention is discussed with reference to the accompanying drawings. 
       FIG. 11  is a diagram depicting the topology of a communications network system including a DNS proxy server in accordance with the second embodiment of the present invention. 
     Compared with the network topology of the first embodiment, there is a difference from the first embodiment in that a plurality of IPv4/IPv6 translators  102  and  103  are connected to the IPv4 network  410 . Nodes and devices corresponding to those shown in the network topology of the first embodiment are assigned the same reference numbers and their explanation is not repeated. 
     The IPv6 network  600  is connected to the IPv4 network  400  via the IPv4/IPv6 translator  101  and also connected to the IPv4 network  410  via the IPv4/IPv6 translator  102  or  103 . 
     Like the IPv4/IPv6 translators  101  and  102 , the IPv4/IPv6 translator  103  is a device for translating IPv4 to IPv6 addressing and vice versa for the destination and source addresses of an IP packet transferred thereto across the network to which it is connected. 
     The IPv4/IPv6 translators  102  and  103  are responsible for different prefix ranges and make IP address translation within the prefix range that each is assigned to handle. 
       FIG. 2  is a block diagram showing the configuration of the DNS proxy server  1   b  in accordance with the second embodiment of the invention. 
     Compared with the DNS proxy server  1   a  of the first embodiment, the DNS proxy server  1   b  differs from the one of the first embodiment in that an IPv4/IPv6 translator information manager  17  has a prefix-network ID reference table  170  to retain mappings between IPv6 prefix values and network IDs and a network ID-translator reference table  180  to retain mappings between network IDs and IPv4/IPv6 translators. 
     Components of the DNS proxy server  1   b  corresponding to those of the DNS proxy server  1   a  of Embodiment are assigned the same reference numbers and their explanation is not repeated. 
       FIG. 13  shows exemplary contents of the prefix-network ID reference table  170 . 
     The prefix-network ID reference table  170  retains mappings between IPv6 prefix values and network IDs, each of which is assigned to each network associated with a prefix value. 
     Specifically, the prefix-network ID reference table  170  consists of a column  171  for prefix entry and a column  172  for network ID entry. Furthermore, the column  171  consists of a field  173  in which a 128-bit IPv6 address value is registered and a field  174  in which the length in bits of the prefix part of the IPv6 address, from the beginning, is registered. 
       FIG. 14  shows exemplary contents of the network ID-translator reference table  180 . 
     The network ID-translator reference table  180  retains mappings between network IDs, each of which is assigned to each network, and the addresses of the IPv4/IPv6 translators, each of which is responsible for address translation for packets carried across each network. 
     Specifically, the network ID-translator reference table  180  consists of a column  181  for network ID entry and a column  182  for IPv4/IPv6 translator information entry. The IPv4/IPv6 translator information column  182  consists of a field  183  in which the IPv6 address of an IPv4/IPv6 translator is registered and a field  184  in which the UDP port number of the IPv4/IPv6 translator is registered. 
     Then, a process for completing a request issued from an IPv6 client  602  for a reverse lookup for an IPv4 client  412  is discussed. 
     Here, in the topology shown in  FIG. 11 , it is assumed that the addresses and names of the clients are initially set as below. 
     The IPv4/IPv6 translator  102  generates, for all devices connected to the IPv4 network  410 , a temporary IPv6 address by adding a prefix “3ffe:2::/96” to the device&#39;s IPv4 address. Likewise, the IPv4/IPv6 translator  103  generates, for all devices connected to the IPv4 network  410 , a temporary IPv6 address by adding a prefix “3ffe:3::/96” to the device&#39;s IPv4 address. 
     Moreover, address translation information on the DNS proxy server  1   b  is registered on the IPv4/IPv6 translator  102  and  103  as well beforehand. 
     In the prefix-network ID reference table  170  held within the DNS proxy server  1   b , both the IPv6 address prefixes “3ffe:2::/96” and “3ffe:3::/96” are mapped to network ID  1 . In the network ID-translator reference table  180 , network ID  1  is mapped to the IPv4/IPv6 translators  102  and  103 , and messages of request for a substitutive address are transmitted to these two translators alternately. 
     Initially, the IPv6 client  602  transmits a message of query for the name of the device having an IPv6 address “3ffe:2::c0a8:2” to the nearest DNS server  601 . The DNS server  601  transfers the query message to the DNS proxy server  1   b , if the server is unable to make the name resolution by itself. 
     Upon receiving the query message, the DNS proxy server  1   b  parses an FQDN “¥[x3ffe00020000000000000000c0a80002/128¥].ip6.arpa” for reverse lookup stored in the question section  22  of the message and extracts the IPv6 address “3ffe:2::c0a8:2” specified by the query sender. 
     Next, the DNS proxy server  1   b  searches the prefix-network ID reference table  170  on the IPv4/IPv6 translator information manager  17 , using the extracted IPv6 address and obtains network ID “1” mapped to the prefix “3ffe.:2::/96” of the IPv6 address of the device about which the query is made by using the query message. Using the thus obtained network ID, the DNS proxy server  1   b  searches the network ID-translator reference table  180 , obtains information of IPv4/IPv6 translators  102  and  103  mapped to the network ID “1”, and selects the IPv4/IPv6 translators  102  and  103  associated with the prefix. 
     Then, the DNS proxy server  1   b  transmits an IP packet of query for a substitutive IPv4 address corresponding to the address “3ffe:2::c0a8:2” to either of the selected two IPv4/IPv6 translators (for example, an IPv4/IPv6 translator  102 ). 
     Upon having received the IP packet (query for a substitutive IPv4 address), the IPv4/IPv6 translator  102  refers to the address translation table to search for an IPv4 address corresponding to the address “3ffe:2::c0a8:2”, obtains “192.168.0.2” as the corresponding IPv4 address, and transmits a reply IP packet J for notification of the substitutive IP address back to the DNS proxy server  1   b.    
     The DNS proxy server  1   b  generates an IPv4 FQDN for reverse lookup from the IPv4 address included in the IP packet (the substitutive IPv4 address as the reply) received from the IPv4/IPv6 translator  102 , replaces the FQDN “¥[x3ffe00020000000000000000c0a80002/128¥].ip6.arpa” for reverse lookup in the question section  22  of the above query message by the generated FQDN “2.0.168.192.in-addr.arpa” for reverse lookup, and transmits a query message in which the QNAME field in the question section contains the converted-to-FQDN to a DNS server  411 . 
     In the course of the message transfer from the IPv6 network  600  to the IPv4 network  410 , the query message with the converted-to-FQDN is converted from an IPv6 packet to IPv4 packet form at the IPv4/IPv6 translator  102  and the IPv4 packet message arrives on the DNS server  411 . 
     Upon having received this message, in response to the message, the DNS server  411  transmits a reply message for notification of a PTR record (domain name) “hostX.v4.net” for the FQDN “2.0.168.192.in-addr.arpa” back to the DNS proxy server  1   b.    
     In the course of the message transfer from the IPv4 network  410  to the IPv6 network  600 , this reply message is converted from an IPv4 packet to IPv6 packet form at the IPv4/IPv6 translator  102  and the IPv6 packet message arrives on the DNS proxy server  1   b . In this message, the QNAME field in the question section  22  contains the IPv4 FQDN “2.0.168.192.in-addr.arpa” for reverse lookup and the NAME field in the answer section  23  contains the pointer to the beginning of the FQDN string in the QNAME field in the question section  22 . 
     Upon receiving the reply message, the DNS proxy server  1   b  parses the FQDN “2.0.168.192.in-addr.arpa” for reverse lookup stored in question section  22  of the message and extracts the IPv4 address “192.168.0.2” of the IPv4 client  412 . 
     Next, the DNS proxy server  1   b  searches the prefix-network ID reference table  170  on the IPv4/IPv6 translator information manager  17 , using the source IPv6 address of the reply to the reverse lookup query and obtains network ID  1  mapped to the prefix “3ffe:2::/96” of the source address of the reply message. Using the thus obtained network ID, the DNS proxy server  1   b  further searches the network ID-translator reference table  180 , obtains information of IPv4/IPv6 translators  102  and  103  mapped to the network ID  1 , and selects an IPv4/IPv6 translator  102  associated with the prefix. 
     The DNS proxy server  1   b  first transmits an IP packet of query for a substitutive IPv6 address corresponding to the address “192.168.0.2” to either of the thus obtained two IPv4/IPv6 translators (for example, an IPv4/IPv6 translator  103 ). 
     The IPv4/IPv6 translator  103  refers to the address translation table to search for an IPv6 address corresponding to the address “192.168.0.22”, but such IPv6 address does not exist in the table. Consequently, the IPv4/IPv6 translator  103  transmits an error response back to the DNS proxy server  1   b.    
     Upon receiving the error response from the IPv4/IPv6 translator  103 , the DNS proxy server  1   b  transmits the IP packet of query for a substitutive IPv6 address corresponding to the address “192.168.0.2” to the other IPv4/IPv6 translator  102 . 
     Upon having received the IP packet (query for a substitutive IPv6 address), the IPv4/IPv6 translator  102  refers to the address translation table to search for an IPv6 address corresponding to the address “192.168.0.2”, obtains “3ffe:2::c0a8:2” as the corresponding IPv6 address, and transmits a reply packet for notification of the substitutive IP address back to the DNS proxy server  1   b.    
     The DNS proxy server  1   b  generates an IPv6 FQDN for reverse lookup from the IPv6 address included in the IP packet (the substitutive IPv6 address as the reply) received from the IPv4/IPv6 translator  102  and replaces the FQDN “2.0.168.192.in-addr.arpa” for reverse lookup in the question section of a message F by the generated FQDN “¥[x3ffe00020000000000000000c0a80002/128¥].ip6.arpa”. The DNS proxy server  1   b  transmits a message F′ in which the QNAME field in the question section contains the converted-to-FQDN and the pointer to the QNAME field has been modified to the IPv6 client  602 . 
     As the result of receiving this message with the replaced FQDN, the IPv6 client  602  obtains information that the name of the device having the IPv6 address “3ffe:2::c0a8:2” is “hostX.v4.net”. 
     Through the above-described process, the reverse DNS lookup on the IPv4 address from the IPv6 network side is complete. 
     In the second embodiment discussed hereinbefore, a reverse lookup request to a DNS server can be completed even in the network topology via a plurality of IPv4/IPv6 translators, as is the case in the first embodiment. Especially, even in a situation where a plurality of IPv4/IPv6 translators which are responsible for different prefix ranges are provided on a same network, a reverse DNS lookup can be performed by reference to the network ID-translator reference table  180  that retains mappings between individual network IDs and IPv4/IPv6 translators. 
     Next, a third embodiment of the invention is discussed with reference to the accompanying drawing. 
       FIG. 15  is a diagram depicting the topology of a communications network system including a node in which a DNS proxy server and IPv4/IPv6 translators are integrated, according to the third embodiment of the present invention. 
     The IPv4 network  410  and IPv6 network  600  are connected via an IPv4/IPv6 gateway  1   c . This IPv4/IPv6 gateway  1   c  incorporates the functionality of the DNS proxy server  1   b  and the functionality of the plurality of IPv4/IPv6 translators  101 ,  102 , and  103  which have been described in the second embodiment of the present invention. Nodes and devices which operate the same as in Embodiments 1 and 2 are assigned the same reference numbers and their explanation is not repeated. 
       FIG. 16  is a block diagram showing the configuration of the IPv4/IPv6 gateway  1   c  of the third embodiment of the present invention. 
     This IPv4/IPv6 gateway  1   c  comprises a DNS proxy server blade  32  which provides functionality equivalent to the functionality of the DNS proxy server  1   b  of the second embodiment, IPv4/IPv6 translator blades  31   a  to  31   n , each providing functionality equivalent to the functionality of an IPv4/IPv6 translator, and switches  33 . 
     On an IPv4/IPv6 translator blade  31 , a program runs to implement the same functionality as the IPv4/IPv6 translator  102  or  103  of the second embodiment. A plurality of IPv4/IPv6 translator blades  31  are installed in the IPv4/IPv6 gateway  1   c  and each translator blade is assigned an IPv4 address and an IPv6 address. 
     On the DNS proxy server blade  32 , a program runs to implement the same functionality as the DNS proxy server  1   b  of the second embodiment. 
     Each blade is a computer device comprising a CPU, storage, and a communication device. 
     A switch  33  routes a packet to an appropriate blade and outputs a processed packet to a network, according to its destination address. 
     IP packets input from each network are routed by the switch to one of the IPv4/IPv6 translator blades  31   a  to  31   n  or the DNS proxy server blade  32 . 
     Subsequent process flow is the same as described in the second embodiment and, therefore, its explanation is not repeated. 
     In the third embodiment configured as described above, by means of a single unit of the IPv4/IPv6 gateway  1   c  including the DNS proxy server  32 , a redundant configuration of the IPv4/IPv6 translators  31   a  to  31   n  can be realized, in addition to the advantageous effect of the second embodiment.