Patent Publication Number: US-2006002407-A1

Title: Network system, network bridge device, network management apparatus, network address assignment method and network address resolution method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2004-196015, filed on Jul. 1, 2004, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1) Field of the Invention  
      The present invention relates to a network system, a network bridge device, a network management apparatus, a network address assignment method and a network address resolution method that prevent bandwidth saturation of a network due to the explosion or concentration of traffic, by reducing traffic at the bridge device between a local area network and a wide area network.  
      2) Description of the Related Art  
      Data transmission networks that comply with the IEEE802.3 international standard, as represented by Ethernet (registered trademark), are increasingly used in many local area networks (LANs) as a native transport mechanism for higher-layer networks in terms of protocol, such as an internet protocol network (hereinafter, “IP network”). In recent years, telecommunication carriers are offering a network called “wide area Ethernet (registered trademark)” that seamlessly relays data from one LAN to another LAN via a wide area network (WAN), and the use of the network is increasing. At the same time, there is a trend towards an outsourcing of enterprise-level network management.  
       FIG. 11  is a schematic of the conventional IP network constructed by such as enterprises. The IP network includes a plurality of LANs  200  and a WAN  201 . The LANs  200  comply with the IEEE802.3 standard. The WAN  201  connects each LAN  200  to enable communication between the LANs  200 , by wide area telecommunication circuits that comply with telecommunications standards other than the IEEE802.3 standard, such as leased lines, frame relay, ATM.  
      In  FIG. 11 , the LANs  200  indicate networks of a branch store A, a branch store B, or a headquarters. Generally, the LANs  200  can be a network, or a plurality of networks connected to one another; whereas the WAN  201  includes, in almost all cases, the LANs  200  connected to one another. In conventional wide area telecommunication circuits, data is transmitted using a data link layer protocol other than the IEEE 802.3. Therefore, routers  204  are arranged as intermediary devices at a boundary between the LANs  200  and the WAN  201 , and used to perform frame conversion between a LAN frame  202  and a WAN frame  203  (in other words, used to complete the transmission using the IEEE 802.3 once). However, when both of the LANs  200  and the WAN  201  comply with the IEEE802.3 standard, the routers  204  are not necessarily needed.  
       FIG. 12  is a schematic of the wide area Ethernet (registered trademark). In  FIG. 12 , LANs  200 A and a WAN  200 B form one virtual LAN (hereinafter, “VLAN”)  210 , and switch devices  211  replace the routers  204  (for example, see Japanese Patent Application Laid-Open No. 2003-169082). Address resolution and IP routing across the entire VLAN  210  are both controlled by a remotely-located network management apparatus (hereinafter, “OSP”: Operation Service Provider)  212  that centrally controls the entire network (in contrast, the conventional IP network shown in  FIG. 11  cannot be centrally controlled because address resolution is performed locally by each LAN  200  and IP routing is performed by each router  204  in the WAN  201 ). Thus, the outsourcing of network management of the LANs  200 A becomes possible.  
      In realty, however, it is difficult for outsourcing service providers to centrally manage the configuration of each LAN  200 A, because the IP network is originally designed to be a highly distributed system and has a high independency. In many cases, each LAN  200 A in the wide area Ethernet (registered trademark) need to be managed by operators staffed to each LAN  200 A, or to support facilities established nearby the LANs  200 A by the provider for the client&#39;s account. This is a reason why the outsourcing of network management is not as cost-effective and widely-diffused as the outsourcing of operations of computers or computer applications. For this reason, in any case of  FIG. 11  or  FIG. 12 , network management remains to be performed by the enterprises, without being outsourced.  
      On the other hand, the network shown in  FIG. 11  has a problem of address resolution protocol (hereinafter, “ARP”) broadcast storm. In  FIG. 11 , an ARP request is broadcast to all the LANs  200  via the WAN  201 . Generally, a common-sense broadcast level for a private LAN of an enterprise is about 0.1% of bandwidth. However, in a network with an extremely large number of LANs, for example 1,000 LANs, the bandwidth is saturated with ARP broadcast frames even if the LANs are symmetrical in terms of traffic. In many cases, however, the LANs are not symmetrical. More specifically, the traffic between one LAN and the headquarters can be a hundred times the traffic between another LAN and the headquarters. Therefore, the ARP broadcast frames from the former LAN causes the saturation of the bandwidth of the latter LAN.  
      In contrast, the ARP broadcast storm does not occur in the network shown in  FIG. 12  because the OSP  212  performs IP routing (in other words, because the broadcast of ARP request is not necessary); conversely, a connection link  213  between the OSP  212  and the WAN  200 B needs to have an extremely large capacity because all the traffic goes back and forth over the connection link  213  to pass through the OSP  212 . In other words, the traffic scalability is virtually narrowed to the bandwidth of this link, while the original function of the switch devises  211  (i.e. distribution of the traffic) being undermined.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to at least solve the problems in the conventional technology.  
      A network system according to an aspect of the present invention includes a network bridge device that is connected to a local area network and a wide area network, wherein the local area network includes a plurality of hosts; and a network management apparatus that is connected to the wide area network and holds management information of the hosts, wherein the network bridge device including a request receiving unit that receives an address assignment request that is broadcast by a host in the local area network; and a request sending unit that unicasts the address assignment request to the network management apparatus.  
      A network system according to another aspect of the present invention includes a network bridge device that is connected to a local area network and a wide area network, wherein the local area network includes a plurality of hosts, wherein the hosts include a first host and a second host; and a network management apparatus that is connected to the wide area network and holds management information of the hosts, wherein the network bridge device including a request receiving unit that receives an address resolution request that is broadcast by the first host; a determining unit that determines whether the first host and the second host that is to respond to the address resolution request belong to same local area network; and a request sending unit that unicasts the address resolution request to the network management apparatus when it is determined by the determining unit that the first host and the second host belong to different local area networks.  
      A network bridge device according to still another aspect of the present invention is connected to a local area network and a wide area network. The local area network includes a plurality of hosts, includes a request receiving unit that receives an address assignment request that is broadcast by a host in the local area network; and a request sending unit that unicasts the address assignment request to the wide area network.  
      A network bridge device according to still another aspect of the present invention is connected to a local area network and a wide area network. The local area network includes a plurality of hosts. The hosts include a first host and a second host, includes a request receiving unit that receives an address resolution request that is broadcast by the first host in the local area network; a determining unit that determines whether the first host and the second host that is to respond to the address resolution request belong to same local area network; and a request sending unit that unicasts the address resolution request to the wide area network when it is determined by the determining unit that the first host and the second host belong to different local area networks.  
      A network management apparatus according to still another aspect of the present invention is connected to a wide area network and holds management information of hosts in a plurality of local area networks that are connected to the wide area network. The network management apparatus includes a request receiving unit that receives an address assignment request that is unicast from the local area network; a registration unit that registers a first layer address that is in the address assignment request and a second layer address to be assigned to a host that sends the address assignment request as the management information of the host; and a response sending unit that unicasts the second layer address to the local area network.  
      A network management apparatus according to still another aspect of the present invention is connected to a wide area network and holds management information of hosts in a plurality of local area networks that are connected to the wide area network. The network management apparatus includes a request receiving unit that receives an address resolution request that is unicast from the local area network; a searching unit that searches the management information of a host that is to respond to the address resolution request; and a response sending unit that unicasts a first layer address in the management information to the local area network.  
      A network address assignment method according to still another aspect of the present invention is executed on a network system that includes a network bridge device that connects a local area network and a wide area network, and a network management apparatus that is connected to the wide area network and holds management information of hosts in the local area network. The network address assignment method includes the network bridge device unicasting the address assignment request that is broadcast by a host in the local area network to the network management apparatus; registering a first layer address that is in the address assignment request and a second layer address to be assigned to the host as the management information of the host; and the network management apparatus unicasting the second layer address to the host.  
      A network address resolution method according to still another aspect of the present invention is executed on a network system that includes a network bridge device that connects a local area network and a wide area network, and a network management apparatus that is connected to the wide area network and holds management information of hosts in the local area network, wherein the hosts includes a first host and a second host. The network address resolution method includes determining whether the first host and the second host that is to respond to an address resolution request that is broadcast by the first host belong to same local area network; the network bridge device unicasting the address resolution request to the network management apparatus when it is determined at the determining that the first host and the second host belong to different local area networks; searching the management information of the second host; and the network management apparatus unicasting an address in the management information to the first host.  
      The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an overall view of a network system configuration according to a first embodiment of the present invention;  
       FIG. 2  is a schematic of a management table managed by an OSP  4  for address resolution;  
       FIG. 3  is a timing chart of management information registration involved in address assignment;  
       FIG. 4  is a flow chart of processing by CPEs  3 A and  3 B in address assignment;  
       FIG. 5  is a flow chart of processing by the OSP  4  in address assignment;  
       FIG. 6  is a timing chart of management information reference involved in address resolution;  
       FIG. 7  is a flow chart of processing by CPEs  3 A and  3 B in address resolution;  
       FIG. 8  is a flow chart of processing by the OSP  4  in address resolution;  
       FIG. 9  is a diagram of frame formats used in the first embodiment of the present invention;  
       FIG. 10  is a diagram for explaining a second embodiment of the present invention;  
       FIG. 11  is a schematic of a conventional IP network;  
       FIG. 12  is a schematic of a wide area Ethernet (registered trademark);  
       FIG. 13  is a block diagram of the functional configuration of the network system according to the first embodiment of the present invention;  
       FIG. 14  is a block diagram of the functional configuration of the network system according to the second embodiment of the present invention; and  
       FIG. 15  is a block diagram of the functional configuration of the network system according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      Exemplary embodiments of a network system, a network bridge device, a network management apparatus, a network address assignment method and a network address resolution method according to the present invention are explained below in reference to the accompanying drawings.  
       FIG. 1  is an overall view of a network system configuration according to a first embodiment of the present invention. The network system shown in  FIG. 1  includes a plurality of local area networks (LANs)  1 A and  1 B that comply with the IEEE802.3 standard; a wide area network (WAN)  2 ; a plurality of network bridge devices  3 A and  3 B (hereinafter, “CPE”: Customer Premises Equipment) that function as gateways to connect the LANs  1 A and  1 B to the WAN  2 ; and a network management apparatus  4  (hereinafter, “OSP”: Operation Service Provider) arranged at a network management center and centrally controls the entire network. The CPE  3 A connects the LAN  1 A to the WAN  2 , and the CPE  3 B connects the LAN  1 B to the WAN  2 . The OSP  4  is connected to the WAN  2  via a switch device (not shown).  
      In the conventional data transmission networks that comply with the IEEE802.3 standard, such as Ethernet (registered trademark), a destination address is usually resolved by broadcasting an address resolution request from one host (hereinafter, “address requiring host”), and sending a response to the request from another host (hereinafter, “address resolving host”) to the address requiring host. In the present invention, however, the OSP  4  sends the response to the address requiring host on behalf of the address resolving host in collaboration with the CPEs  3 A and  3 B, when the address requiring host and the address resolving host are in different LANs  1 A and  1 B connected by the WAN  2 . The OSP  4  functions as both a domain naming system (DNS) server and a dynamic host configuration protocol (DHCP) server to identify active hosts A 1  to A 3  and B 1  to B 3  in the LANs  1 A and  1 B. Information on each host (hereinafter, “management information”) is registered in the OSP  4  through collaboration of the OSP  4  and the CPEs  3 A and  3 B.  
      The LANs  1 A and  1 B, the WAN  2 , and the hosts A 1  to A 3  and B 1  to B 3  respectively have the same functions as those in the existing IP over Ethernet (registered trademark) network.  
      The function of the CPEs  3 A and  3 B is explained next. The CPEs  3 A and  3 B function as standard switch devices, and have their own MAC addresses. Furthermore, the CPEs  3 A and  3 B convert a broadcast frame into an unicast frame, or convert an unicast frame into a broadcast frame. More specifically;  
      1. Address Assignment Request and Response (DHCP)  
      When receiving a DHCP request from the LAN  1 A or  1 B, the CPE  3 A or  3 B unconditionally converts the request, which is a MAC broadcast frame, into a MAC unicast frame to the OSP  4  and sends the request to the WAN  2 . There are many methods of converting the DHCP request into the MAC unicast frame, but in this first embodiment, a destination address (DA) of the DHCP request is converted (overwritten) from “FF : FF : FF : FF : FF : FF” to the MAC address of the OSP  4 .  
      On the other hand, when receiving a DHCP response to the DHCP request from the WAN  2 , the CPE  3 A or  3 B converts the response, which is the MAC unicast frame, into the MAC broadcast frame and broadcasts the response to the LAN  1 A or  1 B.  
      2. Address Resolution Request and Response (ARP)  
      When receiving an ARP request from the LAN  1 A or  1 B, the CPE  3 A or  3 B determines whether the address requiring host and the address resolving host are in the same LAN  1 A or  1 B. When the two hosts are in the same LAN  1 A or  1 B, the CPE  3 A or  3 B does not perform any special action. On the other hand, when the two hosts are not in the same LAN  1 A or  1 B, the CPE  3 A or  3 B converts the request, which is the MAC broadcast frame, into the MAC unicast frame to the OSP  4  and sends the request to the WAN  2 . There are many methods of converting the ARP request into the MAC unicast frame, but in this first embodiment the DA of the ARP request is converted (overwritten) from “FF : FF : FF : FF : FF : FF” to the MAC address of the OSP  4 .  
      On the other hand, when receiving an ARP response to the ARP request from the WAN  2 , the CPE  3 A or  3 B sends the response, which is the MAC unicast frame, to the address requiring host in the LAN  1 A or  1 B.  
      The function of the OSP  4  is explained next. The OSP  4  functions as the general-purpose DHCP server and the DNS server, and creates a management table explained below.  
       FIG. 2  is a schematic of a management table managed by the OSP  4  for address resolution. A management table  10  is a database and functions as a storage means that holds and maintains the management information necessary for the DHCP and the DNS. More specifically, at least a hostname  11 , a first layer address  13  (hereinafter, “MAC address”), and a second layer address  12  (hereinafter, “IP address”) of each of the hosts A 1  to A 3  and B 1  to B 3  are registered in the management table  10  of the OSP  4 . The management information is registered to the management table  10  automatically and in an integrated manner, without changing the standard protocols of networks, such as the IP over Ethernet (registered trademark) network, that comply with the communication procedures prescribed by the IEEE802.3 standard.  
      Furthermore, the OSP  4  sends a special DHCP response to a special DHCP request that is unicast from the CPE  3 A or  3 B and addressed to the OSP  4 . The OSP  4  also searches the management table  10  and sends a standard ARP response to a special ARP request that is unicast from the CPE  3 A or  3 B and addressed to the OSP  4 . More specifically, the OSP  4  has the two functions described below.  
      1. Address Assignment  
      When receiving the DHCP request converted and unicast by the CPE  3 A or  3 B, the OSP  4  unicasts the special DHCP response to the CPE  3 A or  3 B, and the CPE  3 A or  3 B coverts the response to a standard DHCP response that is broadcast in the LAN  1 A or  1 B. At the same time, the OSP  4  adds to the management table  10  a record that includes the hostname, the IP address, and the MAC address of the host that has broadcast the DHCP request in the LAN  1 A or  1 B.  
      2. Address Resolution  
      When receiving the ARP request converted and unicast by the CPE  3 A or  3 B, the OSP  4  sends the standard ARP response to the address requiring host via the CPE  3 A or  3 B. However, the OSP  4  does not perform any special action when the record of the address resolving host is not found in the management table  10 .  
      Sequences of address assignment and address resolution in the network system that has the above configuration are explained next.  
      1. Sequence of Address Assignment  
      It is assumed that in  FIG. 1  the host A 1  is newly connected to the LAN  1 A.  FIG. 3  is a timing chart of management information registration involved in address assignment. When the host A 1  broadcasts a standard DHCP (address assignment) request to the LAN  1 A (step D 1 ), the CPE  3 A converts (overwrites) the DA of the request (step D 2 ) and unicasts the request over the WAN  2  to the OSP  4  (step D 3 ). The OSP  4  determines the management information by allocating the IP address to the host A 1 , and registers the IP address and the MAC address of the host A 1  in the record of the management table  10  whose hostname  11  corresponds to that of the host A 1  (step D 4 ). Then, the OSP  4  unicasts the DHCP response including the allocated IP address to the host A 1  (step D 5 ). Each step D 1  to D 5  is also shown in  FIG. 1 .  
       FIG. 4  is a flow chart of processing by the CPEs  3 A and  3 B in address assignment. Each step in  FIG. 4  is explained with reference to  FIG. 13 , which is a block diagram of the functional configuration of the network system according to the first embodiment of the present invention.  
      The CPEs  3 A and  3 B are in a standby mode to receive the DHCP request or the DHCP response (step S 1 ). When a receiving unit  1300  of the CPEs  3 A and  3 B receives the DHCP request or the DHCP response (step S 2 ), the receiving unit  1300  determines whether the receiving port is a LAN port (step S 3 ). When the receiving port is the LAN port (in other words, when the CPEs  3 A and  3 B have received the DHCP request from the LAN  1 A or  1 B) (step S 3 : Yes), a sending unit  1301  of the CPEs  3 A and  3 B converts (overwrites) the DA of the request to the MAC address of the OSP  4  (in other words, converts the request into the MAC unicast frame to the OSP  4 ) (step S 4 ). Then, the sending unit  1301  of the CPEs  3 A and  3 B sends the request to a WAN port (step S 5 ).  
      On the other hand, when the receiving port is not the LAN port (in other words, when the CPEs  3 A and  3 B have received the DHCP response from the WAN  2 ) (step S 3 : No), the sending unit  1301  of the CPEs  3 A and  3 B converts (overwrites) the DA of the response to “FFFFFF” (step S 6 ), and sends the response to the LAN port (step S 7 ). After the successive processing is completed by the execution of steps S 5  or S 7 , the CPEs  3 A and  3 B return to step S 1  and enter the standby mode again.  
       FIG. 5  is a flow chart of processing by the OSP  4  in address assignment. Each step in  FIG. 5  is explained with reference to  FIG. 13 .  
      The OSP  4  is in a standby mode (step S 11 ). When a receiving unit  1303  of the OSP  4  receives the DHCP request unicast from the CPE  3 A or  3 B (step S 12 ), a registration unit  1304  of the OSP  4  allocates the IP address to the host that has broadcast the DHCP request in the LAN  1 A or  1 B, and registers the IP address and the MAC address of the above host in the record of the management table  10  whose hostname  11  corresponds to that of the above host (step S 13 ). Then, a sending unit  1305  of the OSP  4  creates the DHCP response (step S 14 ), and unicasts the response from the WAN port to the above host via the CPE  3 A or  3 B (step S 15 ). After execution of the successive processing above, the OSP  4  returns to step S 11  and enters the standby mode again.  
      2. Sequence of Address Resolution  
      When the host A 3  broadcasts the ARP request (step D 6  in  FIG. 1 ) to find the MAC address of the host A 1 , which has participated in the same LAN  1 A according to the sequence explained above (steps D 1  to D 5  in  FIGS. 1 and 3 ), the CPE  3 A does not perform any special action, and the host A 1  sends the standard ARP response to the host A 3  (step D 7  in  FIG. 1 ).  
      On the other hand,  FIG. 6  is a timing chart of management information reference involved in address resolution when the host B 3  in the LAN  1 B tries to communicate with the host A 1  in the different LAN  1 A. The host B 3  broadcasts the ARP request to the LAN  1 B (step D 11 ). The CPE  3 B converts (overwrites) the DA of the request, in other words, converts the MAC broadcast frame into the MAC unicast frame (step D 12 ). Then, the CPE  3 B asks the OSP  4  to reply the MAC address of the host A 1  by unicasting the ARP request to the OSP  4  via the WAN  2  (step D 13 ). The OSP  4  refers to the management table  10  shown in  FIG. 2 , and searches the MAC address  13  corresponding to the IP address set in the request, namely the MAC address of the host A 1  (step D 14 ). Then, the OSP  4  sends the ARP response including the MAC address of the host A 1  to the host B 3  via the CPE  3 B (step D 15 ). Each sequence D 11  to D 15  is also shown in  FIG. 1 .  
      After address resolution according to the sequence explained above, the host B 3 , which has received the ARP response broadcast in the LAN  1 B, initiates host-to-host communication using the MAC unicast frame with the host A 1  in the LAN  1 A (step D 16 ).  
       FIG. 7  is a flow chart of processing by CPEs  3 A and  3 B in address resolution. Each step in  FIG. 7  is explained with reference to  FIG. 13 .  
      The CPEs  3 A and  3 B are in a standby mode to receive the ARP request or the ARP response (step S 21 ). When the receiving unit  1300  of the CPEs  3 A and  3 B receives the ARP request or the ARP response (step S 22 ), the receiving unit  1300  determines whether the receiving port is the LAN port (step S 23 ). When the receiving port is the LAN port (in other words, when the CPE  3 A and  3 B have received the ARP request from the LAN  1 A or  1 B) (step S 23 : Yes), a determining unit  1302  of the CPEs  3 A and  3 B determines whether the address requiring host and the address resolving host are in the same LAN  1 A or  1 B (step S 24 ). When the two hosts are not in the same LAN  1 A or  1 B (in other words, when the MAC address of the address resolving host is unknown to the CPE  3 A or  3 B) (step S 24 : Yes), the sending unit  1301  of the CPEs  3 A and  3 B converts (overwrites) the DA of the request to the MAC address of the OSP  4  (step S 25 ) and sends the request to the WAN port (step S 26 ).  
      On the other hand, when the receiving port is not the LAN port (in other words, when the CPE  3 A and  3 B have received the ARP response from the WAN  2 ) (step S 23 : No), the sending unit  1301  of the CPEs  3 A and  3 B sends the response to the LAN port (step S 27 ). When the two hosts are in the same LAN  1 A or  1 B (step S 24 : No), the CPE  3 A or  3 B returns to step S 21 . After the successive processing is completed by the execution of steps S 26  or S 27 , the CPE  3 A and  3 B return to step S 21  and enter the standby mode again.  
       FIG. 8  is a flow chart of processing by the OSP  4  in address resolution. Each step in  FIG. 8  is explained with reference to  FIG. 13 .  
      The OSP  4  is in a standby mode (step S 31 ). When the receiving unit  1303  of the OSP  4  receives the ARP request unicast from the CPE  3 A or  3 B (step S 32 ), a searching unit  1306  of the OSP  4  searches the management table  10  shown in  FIG. 2  (step S 33 ) and determines whether there is the record of the address resolving host (step S 34 ). When there is not the record (step S 34 : No), the OSP  4  returns to step S 31 . When there is the record (step S 34 : Yes), the sending unit  1305  of the OSP  4  creates the ARP response including the MAC address of the address resolving host (step S 35 ) and sends the response from the WAN port to the address requiring host via the CPE  3 A or  3 B (step S 36 ). When the successive processing is completed by the execution of step S 36 , the OSP  4  returns to step S 31 , and enters the standby mode again.  
       FIG. 9  is a diagram of frame formats used in the first embodiment of the present invention. Each frame format for processes D 1 , D 3 , D 5 , D 11 , D 13 , and D 15  in  FIGS. 1, 3 , and  6  are shown in  FIG. 9 . In  FIG. 9 , “000AEB849C33” is the MAC address of the OSP  4 , and “000038EA9F41” is the MAC address of the host A 1 . As shown in  FIG. 9 , standard DHCP and ARP sequences are implemented between the hosts A 1  to A 3  and B 1  to B 3  and the CPEs  3 A and  3 B. On the other hand, the sequences between the CPEs  3 A and  3 B and the OSP  4  are implemented by the MAC unicast frame converted from the MAC broadcast frame, in stead of the MAC broadcast frame which is conventionally used for the sequences.  
      According to the first embodiment explained above, it is possible to obtain the following two effects;  
      1. Prevention of the ARP Broadcast Storm  
      In the network system described above, the broadcast frames, which have caused the above-mentioned problems in conventional arts, are classified at the CPEs  3 A and  3 B into (a) ARP broadcast frames transferred within a single LAN, (b) ARP broadcast frames converted into the MAC unicast frames and transferred over a plurality of LANs, and (c) other broadcast frames transferred similarly to conventional arts (the avove (c) does not become a serious problem because almost all the broadcast frames are ARP broadcast frames). Accordingly, the problem of the saturation of bandwidth by the ARP broadcast frames is resolved, while the same seamlessness as conventional arts being maintained.  
      2. Prevention of Traffic Concentration to the OSP  4   
      In the network system described above, all the unicast frames from a host in a LAN to another host in a different LAN are transferred without passing through the OSP  4 . The frames sent or received by the OSP  4  are only the DHCP request and response used to register the host, and the ARP request and response used to identify the MAC address of the host. As a result, normally more than 99.9% of the frames is switched to a route without passing through the OSP  4 . This results in a prevention of traffic concentration to the OSP  4 .  
      Moreover, the OSP  4  can centrally manage the configuration of network system because the addition of new hosts is managed by DHCP server, combined with the DNS server, mounted to the OSP  4 . Due to the possession of the management table  10  explained above, the OSP  4  can synchronize the information for switching and the information on network configuration, and can detect the majority of the information necessary for a fault recovery.  
      Furthermore, an incremental transfer of network management becomes possible, and the outsourcing of network management can be implemented flexibly. For example, when the network management of a private IP routing network that includes a plurality of LANs and a WAN is outsourced, at first only the LANs adjoining the WAN are reconstructed according to the first embodiment. At this stage, only the network management for the WAN and the LAN adjoining the WAN can be outsourced. Later, the network management for other LANs connected to the above LANs by IP routers can be outsourced by replacing the IP routers with switch devices.  
      A second embodiment of the present invention is explained next with reference to  FIG. 14 , which is a block diagram of the functional configuration of the network system according to the second embodiment.  
      In principle, each physical or logical interface of an IP host must be assigned a unique IP address. However, when the OSP  4  accommodates a plurality of IP networks of different managing bodies, there can be a plurality of interfaces that are assigned an identical IP address.  
       FIG. 10  is a diagram for explaining the second embodiment of the present invention. As shown in  FIG. 10 , two private networks  101  and  102  are connected to an OSP  41 . In the network  101 , LANs  1 A and  1 B are respectively connected to a WAN  2 A via CPEs  3 A and  3 B. In the network  102 , LANs  1 C and  1 D are respectively connected to a WAN  2 B via CPEs  3 C and  3 D. The network  101  includes a plurality of hosts A 1  to A 3  and B 1  to B 3 . The network  102  includes a plurality of hosts C 1  to C 3  and D 1  to D 3 .  
      If the OSP  4  according to the first embodiment is introduced as-is to the network shown in  FIG. 10 , the OSP  4  cannot identify which host should receive the ARP response because there are two hosts assigned an identical IP address in the two networks  101  and  102 . As a result, the OSP  4  cannot send the ARP response appropriately, in other words, the OSP  4  can disadvantageously send the response to an irrelevant host as well as a relevant host.  
      For this reason, the OSP  41  according to the second embodiment, which includes a plurality of physical or logical interfaces for connecting the networks  101  and  102  respectively to the OSP  41 , manages address spaces of the network  101  and  102  respectively and associates each address space with the number of each interface. The OSP  41  includes a plurality of management tables  10   a  and  10   b  (shown in  FIGS. 10 and 14 ) that correspond to the networks  101  and  102  respectively, and store the addresses of hosts A 1  to B 3  and C 1  to D 3  separately. A sending unit  1405  of the OSP  41  (shown in  FIG. 14 ) sends a response (such as the DHCP response and the ARP response) to a certain IP address only from one interface that has received a request (such as the DHCP request and the ARP request), even if there are two hosts of the address in the networks  101  and  102 . In other words, the OSP  41  sends the response only to one host, regardless of the destination address of the response.  
      According to the second embodiment, the OSP  41  can accommodate a plurality of networks, in other words, can send the ARP response only to the relevant host that has sent the ARP request. Furthermore, the cost of outsourcing reduces because the facilities can be consolidated without increasing the number of the OSP  41  and because services for a plurality of enterprises are centrally performed by the OSP  41 .  
      A third embodiment of the present invention is explained next with reference to  FIG. 15 , which is a block diagram of the functional configuration of the network system according to the third embodiment.  
      The third embodiment relates to the processing when a host is disconnected or is moved from one LAN to another LAN. The disposition of the hosts A 1  to A 3  and B 1  to B 3  in the LANs  1 A and  1 B shown in  FIG. 1  can change due to a failure, a circuit disconnection, a movement, and the like. Whenever there is a change of the disposition, an OSP  42  (shown in  FIG. 15 ) manages an incorporation of the changes in the management table  10 .  
      1. When the Host Becomes Disconnected  
      As shown in  FIG. 15 , the OSP  42  has a detecting unit  1507  that detects disconnection of the hosts A 1  to A 3  and B 1  to B 3 . The hosts A 1  to A 3  and B 1  to B 3  can be connected to the LAN  1 A or  1 B directly or indirectly (via a cascade bridge, for example). If there is the host connected indirectly to the LAN  1 A or  1 B, the connection status of the host is previously set in the CPE  3 A or  3 B. The detecting unit  1507  of the CPEs  3 A and  3 B periodically sends (1) a pulse or the like to the directly-connected hosts, as well as (2) an activity confirmation signal to the indirectly-connected hosts, to detect the connection status of the hosts A 1  to A 3  and B 1  to B 3 . When the detecting unit  1507  of the CPE  3 A detects the disconnection of the host A 1 , for example, a sending unit  1501  of the CPE  3 A sends to the OSP  42  a notice that instructs the OSP  42  to delete the management information of the host A 1  in the management table  10  (hereinafter, “deregistration notice”). When a receiving unit  1503  of the OSP  42  receives the deregistration notice, a registration unit  1504  of the OSP  42  deletes the IP address  12  and the MAC address  13  of the host A 1  in the management table  10  shown in  FIG. 2 . As a result, the OSP  42  can continuously maintain the information on the disposition of the hosts A 1  to A 3  and B 1  to B 3  in the most up-to-date status.  
      2. When the Host Duplicates the Sending of DHCP Request  
      It is assumed that the MAC address  13  of the host A 1  is already registered in the management table  10  of the OSP  42 . When the host A 1  is moved from the LAN  1 A to the LAN  1 B, the deregistration processing of the host A 1  can be not performed correctly for some reason, such as a failure, a movement, and the like. As a result, the OSP  42  receives the DHCP request with the MAC address that is already registered in the management table  10 , when the host A 1  sends the DHCP request to the OSP  42  via the LAN  1 B. Upon receiving the DHCP request, a registration unit  1504  (shown in  FIG. 15 ) of the OSP  42  deletes the previously registered management information of the host A 1 , creates new management information of the host A 1  as a new host on the LAN  1 B, and sends the DHCP response to the CPE  3 B.  
      According to the third embodiment, even if the deregistration is not excecuted correctly, unnecessary old management information is deleted; and new management information that corresponds to the new disposition is created, registered, and managed.  
      The network address assignment method and the network address resolution method explained above are executed on computers such as personal computers, workstations, and the like that use a computer program that is prepared in advance of use. This computer program is recorded on recordable media that can be read by computers, such as hard disks, flexible disks, CD-ROMs, MOs, DVDs, and the like. It is also acceptable for the computer program to be a transmittable medium that is distributed via a network such as the Internet and the like.  
      A network system, a network bridge device, a network management apparatus, a network address assignment method and a network address resolution method according to the present invention have the effects of centrally controlling an entire network system comprised of a plurality of local area networks and a wide area network, and facilitating an outsourcing of network management.  
      Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.