Patent Publication Number: US-8995442-B2

Title: Apparatus and method for translating an address of a packet transferred between networks

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2011-168556, filed on Aug. 1, 2011, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to apparatus and method for translating an address of a packet transferred between networks. 
     BACKGROUND 
     NAT (Network Address Translation) devices have been known each of which performs translation between a network address used on one network and a network address used on another network, at a connection point connecting different networks. In addition, multi-homed networks have been known each of which is connected to another network through a plurality of connection points. 
     In addition, a communication method has been known where when a terminal movable among different networks performs packet transmission between any different networks, a transmission source address of a packet transmitted by the terminal is translated into a same global address. In a first network, a first address change mechanism is provided that changes the transmission source address of a packet transmitted from the terminal, to the global address of a packet. In addition, a second address change mechanism is provided in a second network, and when the terminal has moved from the first network to the second network, the second address change mechanism changes the transmission source address of a packet transmitted from the terminal, to the same address as the global address. The second address change mechanism detects that a registration request has been transmitted from the terminal to a mobile IP agent of the second network, and after the detection, the second address change mechanism acquires the global address from the first address change mechanism. Alternatively, the second address change mechanism detects that a registration response has been transmitted from a mobile IP agent of the first network to the mobile IP agent of the second network, and after the detection, the second address change mechanism acquires the global address from the first address change mechanism. 
     An address translation method has been known where in a home network, a home agent and an address translation function unit are provided and in a foreign network of a movement destination, a foreign agent and an address translation function unit are provided. The foreign network inquires of the agent of the home network about the global address of a mobile terminal that has moved to the foreign network, and stores the global address of the mobile terminal in the address translation function unit. Examples of such methods include techniques disclosed in Japanese Laid-open Patent Publication No. 2001-274834 and Japanese Laid-open Patent Publication No. 2002-94546. 
     SUMMARY 
     According to an aspect of the invention, there is provided an apparatus for translating an address of a packet transferred between networks. The apparatus translates a destination address of a first packet that is transferred from an external communication device in an external network to an internal communication device in an internal network through the external network, from an external-network address that has been assigned to the internal communication device using an addressing scheme for the external network, to an internal-network address that has been assigned to the internal communication device using an addressing scheme for the internal network. The apparatus transmits, to another apparatus, address information including the external-network address and identifier information for identifying the external communication device where the another apparatus is configured to translate a source address of a second packet that is transferred from the internal communication device to the external communication device, from the internal-network address to the external-network address. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a configuration example of a communication system, according to an embodiment; 
         FIG. 2  is a diagram illustrating an example of an internal host management table, according to an embodiment; 
         FIG. 3  is a diagram illustrating an example of a hardware configuration of an address translation device, according to an embodiment; 
         FIG. 4  is a diagram illustrating a configuration example of an address translation device, according to a first embodiment; 
         FIG. 5  is a diagram illustrating an example of an address translation table, according to a first embodiment; 
         FIG. 6  is a diagram illustrating an example of movement information, according to a first embodiment; 
         FIG. 7  is a diagram illustrating an example of extracted NAT information, according to a first embodiment; 
         FIG. 8  is a schematic diagram illustrating an example of a packet flow when an internal host has moved, according to a first embodiment; 
         FIGS. 9A to 9C  are diagrams each illustrating an example of an address translation table, according to a first embodiment; 
         FIG. 10  is a diagram illustrating an example of a sequence of communication operations, according to a first embodiment; 
         FIG. 11  is a diagram illustrating an example of an operational flowchart for a first address translation device, according to a first embodiment; 
         FIG. 12  is a diagram illustrating an example of an operational flowchart for a second address translation device, according to a first embodiment; 
         FIG. 13  is a diagram illustrating an example of an address translation table, according to a second embodiment; 
         FIG. 14  is a diagram illustrating an example of a pool address table, according to a second embodiment; 
         FIG. 15  is a schematic diagram illustrating an example of a packet flow when an internal host moves, according to a second embodiment; 
         FIGS. 16A to 16C  are diagrams each illustrating an example of an address translation table, according to a second embodiment; 
         FIG. 17  is a diagram illustrating an example of extracted NAT information, according to a second embodiment; 
         FIG. 18  is a diagram illustrating an example of an operational flowchart performed by a first address translation device, according to a second embodiment; 
         FIG. 19  is a schematic diagram illustrating an example of a packet flow when an internal host moves, according to a second embodiment; 
         FIG. 20  is a diagram illustrating an example of an address translation table, according to a third embodiment; 
         FIG. 21  is a schematic diagram illustrating an example of a packet flow when an internal host moves, according to a third embodiment; 
         FIGS. 22A to 22C  are diagrams each illustrating an example of an address translation table, according to a third embodiment; 
         FIG. 23  is a diagram illustrating an example of extracted NAT information, according to a third embodiment; 
         FIG. 24  is a diagram illustrating an example of an address translation table, according to a fourth embodiment; 
         FIG. 25  is a diagram illustrating an example of a pool address table, according to a fourth embodiment; 
         FIG. 26  is a diagram illustrating a configuration example of an address translation device, according to a fifth embodiment; 
         FIG. 27  is a diagram illustrating an example of a mobile host table, according to an embodiment; 
         FIG. 28  is a schematic diagram illustrating an example of a packet flow, according to a fifth embodiment; 
         FIGS. 29A to 29C  are diagrams each illustrating an example of an address translation table, according to a fifth embodiment; 
         FIG. 30  is a diagram illustrating an example of an address translation table, according to a sixth embodiment; and 
         FIG. 31  is a diagram illustrating an example of movement information, according to a sixth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following description, a multi-homed network is expressed as an “internal network”, and a network connected to the multi-homed network is expressed as an “external network”. In addition, an address that is assigned using an addressing scheme for the internal network will be expressed as an “internal address” or “internal-network address”, and an address that is assigned using an addressing scheme for the external network will be expresses as an “external address” or “external-network address”. In addition, a host in the internal network will be expressed as an “internal host” or “internal communication device”, and a host establishing communication with the internal host through the external network will be expressed as an “external host” or “external communication device”. 
     With respect to each internal host in the internal network, one of a plurality of connection points is designated as a connection point through which a packet transmitted from the internal host is to transit. In the present specification, such a connection point is expressed as a “transit connection point”, in some cases. An example of the transit connection point is a default gateway. 
     When the transit connection point of an internal host has been changed, a case where it is difficult to continue communication with an external host occurs in some cases, for the following reasons. When the change of the transit connection point has occurred, the external address of the internal host is changed, from an address assigned to the transit connection point before the change, to an address assigned to a transit connection point after the change. A Domain Name System (DNS) server updates the external address of the internal host. Since it takes a certain amount of time for the update to be transmitted to the external host, a case occurs where, using the external address before the change, the external host transmits a packet after the update of the external address. This packet reaches the transit connection point before the change. 
     At this time, if the internal host remains in an address translation table, the transit connection point before the change translates the destination address of the packet into the internal address. Accordingly, the packet reaches the internal host. The internal host sends back, to the external host, a response packet corresponding to this packet. This response packet is transmitted to the external network through the transit connection point after the change. 
     Therefore, the transmission source address of the response packet received by the external host does not match the transmission destination address of the packet transmitted by the external host. Since it is difficult for a host within the external network to identify the transmitted packet and the response packet as packets transmitted and received in the same sequence of communication operations, the host fails to continue communication. 
     &lt;1. Whole Configuration of Communication System&gt; 
     Hereinafter, embodiments of the present technology will be described with reference to attached drawings. 
       FIG. 1  is a diagram illustrating a configuration example of a communication system, according to an embodiment. A communication system  1  includes an internal network  2 , a first address translation device  3   a  to a third address translation device  3   c , a first internal host  4   a , a second internal host  4   b , and a host management server  5 . In addition, in the following description, the first internal host  4   a  and the second internal host  4   b  will be also collectively expressed as “internal hosts  4 ”. 
     The internal network  2  is a multi-homed network connected to an external network  90  through a plurality of connection points. A reference numeral  91  in the drawing indicates an external host establishing communication with the internal host  4  through the external network  90 . A DNS server  92  includes a DNS record  93  serving as a correspondence table between the domain name and the external address of the internal host  4 , and is able to provide the external address of the internal host  4  in response to an inquiry from the external host  91 . An example of the external network  90  is the Internet. 
     Hereinafter, the external host  91  and the internal host  4  will be also expressed as “an external communication device” and “an internal communication device”, respectively. 
     The first address translation device  3   a  to the third address translation device  3   c  individually translate the addresses of packets transiting through individual connection points, in connection points different from one another. In the attached drawings, the first address translation device, the second address translation device, and the third address translation device are expressed as “NAT 1 .”, “NAT 2 ”, and “NAT 3 ”, respectively. In addition, in the following description, the first address translation device  3   a  to the third address translation device  3   c  will be also collectively expressed as “address translation devices  3 ”. 
     The host management server  5  includes an internal host management table  6  serving as a correspondence table that stores information on a correspondence relationship between internal hosts  4  and the address translation devices  3  used for address translation of transmitted packets of the internal hosts  4 . 
       FIG. 2  is a diagram illustrating an example of an internal host management table, according to an embodiment. The internal host management table  6  includes an information element “internal host” and an information element “address translation device”. The information element “internal host” indicates an identifier identifying each internal host  4  within the internal network  2 . The information element “address translation device” indicates address information for the address translation device  3  performing address translation for a transmitted packet of each internal host  4 . The illustrated example indicates that the first address translation device  3   a  is used for address translation for a transmitted packet of the first host  4   a  and the second address translation device  3   b  is used for address translation for a transmitted packet of the second host  4   b.    
     Processing will be described that is performed in the host management server  5  when the transit connection point of the internal host  4  is changed. The transit connection point may be changed, for example, when the internal host  4  operating as a virtual machine on a physical computer within the internal network  2  is moved so as to operate on another physical computer within the internal network  2 . In addition, the transit connection point may be changed, for example, when, within the internal network  2 , the position of the internal host  4  on a network topology changes in association with the physical movement of the internal host  4 . In the following description, a state in which the transit connection point of the internal host  4  is changed will be also expressed as “the internal host  4  is moved”. In addition, the transit connection point may also be changed, for example, when the default gateway of the internal host  4  is simply changed. 
     When the internal host  4  is moved, an address translation device that performs address translation for a transmitted packet of the internal host  4  after movement will be also expressed as a “post-change address translation device” in the following description. In addition, an address translation device that performs address translation for a transmitted packet of the internal host  4  before the movement will be also expressed as a “pre-change address translation device”. 
     When the internal host  4  is moved, the host management server  5  updates the external address that has been assigned to the moved internal host  4  and registered in the DNS record  93 , to an external address to be translated by the post-change address translation device  3 . In addition, the host management server  5  updates the default gateway of the moved internal host  4  to the post-change address translation device  3 . 
     In the following description, it is assumed that the internal addresses of the first internal host  4   a  and the second internal host  4   b  are “A” and “B”, respectively. In addition, the first address translation device  3   a  assigns an external address “a 1 ” to the first internal host  4   a , and the second address translation device  3   b  assigns an external address “b 2 ” to the second internal host  4   b . In addition, the external address of the external host  91  is “c”. 
     &lt;2. Hardware Configuration of Address Translation Device&gt; 
     Next, a configuration of the address translation device  3  will be described. 
       FIG. 3  is a diagram illustrating an example of a hardware configuration of an address translation device, according to an embodiment. The address translation device  3  includes a processor  10 , an auxiliary storage device  11 , a memory  12 , and a communication interface  13 . These configuration elements  10  to  13  are electrically connected to one another via a bus  14 . In addition, the hardware configuration illustrated in  FIG. 3  is just one example of a configuration for realizing the address translation device  3 . Any hardware configuration may be adopted that allows processing described in the following description to be performed. 
     An address translation program  15  and data  16  are stored in the auxiliary storage device  11 . The auxiliary storage device  11  may include a storage element such as a nonvolatile memory, a hard disk, or a read only memory (ROM). By executing the address translation program  15  stored in the auxiliary storage device  11 , the processor  10  executes processing relating to address translation for a packet, which will be described later. In addition, for example, as the data  16 , an address translation table  17  used for address translation in the address translation device  3  may be stored in the auxiliary storage device  11 . 
     In the memory  12 , a running program executed by the processor  10  and data to be temporarily used by this program may be stored. The memory  12  may include a random access memory (RAM). The communication interface  13  performs processing for transmitting or receiving a packet between the internal network  2  and the external network  90 . 
     The address translation device  3  may include a removable medium reading unit (not depicted in  FIG. 3 ) used for reading data stored in a computer-readable portable recording medium. The removable medium reading unit may be an access device, for example, for a CD-ROM drive device, a DVD-ROM drive device, a flexible disk drive device, a CD-R drive device, a DVD-R drive device, an MO drive device, or a flash memory device. In an embodiment, the address translation program  15  may be distributed with being stored in a computer-readable portable recording medium, and may be installed from the removable medium reading unit to the auxiliary storage device  11 . In another embodiment, the address translation program  15  may also be read into the address translation device  3  through the communication interface  13  and installed to the auxiliary storage device  11 . 
     &lt;3. First Embodiment of Address Translation Device&gt; 
       FIG. 4  is a diagram illustrating a configuration example of an address translation device, according to a first embodiment. The address translation device  3  may be configured to include transmission and reception units  21  and  24 , an external packet identification unit  22 , an external address translation unit  23 , an internal packet identification unit  25 , and an internal address translation unit  26 . In addition, the address translation device  3  may include an identifier designation unit  27 , a NAT information extraction unit  28 , and a NAT information addition unit  29 . 
     In accordance with the address translation program  15  stored in the auxiliary storage device  11 , the processor  10  in  FIG. 3  preferably performs a cooperative operation with another hardware element in the address translation device  3  on an as-needed basis, and hence performs information processing due to each configuration element illustrated in  FIG. 4 . For example, the processor  10  executes information processing due to the external packet identification unit  22 , the external address translation unit  23 , the internal packet identification unit  25 , the internal address translation unit  26 , the identifier designation unit  27 , the NAT information extraction unit  28 , and the NAT information addition unit  29 . In addition, signal processing due to the transmission and reception units  21  and  24  may be executed by the communication interface  13 . 
     The transmission and reception unit  21  performs processing for receiving a packet from the external network  90  and processing for transmitting a packet to the external network  90 . The external packet identification unit  22  identifies the type of a received packet from the external network  90 . The external packet identification unit  22  outputs a data packet received from the external network  90 , to the identifier designation unit  27 . 
     The packet that has transited through the identifier designation unit  27  is inputted to the external address translation unit  23 . In accordance with the address translation table  17 , the external address translation unit  23  translates the external address of the inputted packet into an internal address. 
       FIG. 5  is a diagram illustrating an example of an address translation table, according to a first embodiment. 
     An address translation table  17  includes, as information elements, “the internal address of an internal host”, “the external address of the internal host”, and “the external address of an external host”. The information element “the internal address of the internal host” indicates the internal address of the internal host  4 , and the information element “the external address of the internal host” indicates an external address assigned to the internal host  4  by the address translation device  3 . The information element “the external address of the external host” is the external address of an external host serving as the transmission source of a packet transmitted to the internal host  4  through the address translation device  3 . 
     The illustrated example indicates that the address translation device has assigned the external address “a 1 ” to the first internal host  4   a  having the internal address “A” and has routed, through the address translation device, a packet whose transmission source is the external host  91  having the external address “c” and whose destination is the first internal host  4   a.    
     The external address translation unit  23  searches the address translation table  17  for an entry matching the combination of the transmission source address and the transmission destination address that have been set to a packet using external addresses. The external address translation unit  23  translates the external address of the internal host  4 , which serves as the transmission destination address of the packet, into the internal address of the internal host that is being stored in the matched entry of the address translation table  17 . The external address translation unit  23  outputs, to the transmission and reception unit  24 , the packet whose address has been translated. 
     The transmission and reception unit  24  performs processing for receiving a packet from the internal network  2  and processing for transmitting a packet to the internal network  2 . The packet whose address has been translated by the external address translation unit  23  is transmitted to the internal network  2  by the transmission and reception unit  24 . The internal packet identification unit  25  identifies the type of a packet received from the internal network  2 . The internal packet identification unit  25  outputs the packet received from the internal network  2  to the internal address translation unit  26 . 
     In addition, when the received packet is a packet conveying movement information and extracted NAT information which will be described later, the internal packet identification unit  25  outputs the movement information and the extracted NAT information to the NAT information extraction unit  28  and the NAT information addition unit  29 , respectively. Further, movement information and extracted NAT information may also be transmitted to the address translation device  3  through the external network  90 . In this case, the external packet identification unit  22  may identify a packet conveying the movement information and the extracted NAT information and output the movement information and the extracted NAT information to the NAT information extraction unit  28  and the NAT information addition unit  29 , respectively. 
     In accordance with the address translation table  17 , the internal address translation unit  26  translates the internal address of an inputted packet to an external address. The internal address translation unit  26  searches the address translation table  17  for an entry matching the combination of the internal address of the internal host  4 , which serves as the transmission source address of a packet, and the external address of the external host  91 , which serves as the transmission destination address thereof. The internal address translation unit  26  translates the internal address of the internal host  4 , which is designated as the transmission source address of the packet, into the external address of the internal host that is being stored in the matched entry of the address translation table  17 . The internal address translation unit  26  outputs, to the transmission and reception unit  21 , the packet whose address has been translated. The packet whose address has been translated by the internal address translation unit  26  is transmitted to the external network  90  by the transmission and reception unit  21 . 
     The identifier designation unit  27 , upon receiving a packet from the external network  90 , determines whether or not an entry corresponding to the transmission source address and the transmission destination address of the received packet exists in the address translation table  17 . When the corresponding entry does not exist in the address translation table  17 , the identifier designation unit  27  creates a new entry storing therein the external address and the internal address of the internal host  4  serving as a transmission destination and the external address of the external host  91 , and adds the created new entry to the address translation table  17 . 
     The external address of the external host  91 , which is stored in the new entry, corresponds to the information element “the external address of an external host” in the address translation table  17 . As described above, the information element “the external address of an external host” indicates the transmission source address of a packet having transited through the address translation device  3 . Therefore, the external address of the external host  91 , which is stored as the information element “the external address of the external host”, is used as an identifier identifying a state in which a packet from the external host  91  has transited through the address translation device  3 . 
     The NAT information extraction unit  28 , upon receiving movement information, creates extracted NAT information. Here, the movement information is information identifying an internal host  4  that has moved after transmitting a packet whose address translation being performed by the address translation device  3 . 
       FIG. 6  is a diagram illustrating an example of movement information, according to a first embodiment. The movement information includes an identifier identifying the internal host  4  to move, the internal address of the internal host  4  to move, and the address of the post-change address translation device  3 . 
     The NAT information extraction unit  28  extracts, from the address translation table  17 , entries including the address of the internal host  4  that is specified by the movement information, and creates the extracted NAT information storing therein the extracted entries. 
       FIG. 7  is a diagram illustrating an example of extracted NAT information, according to a first embodiment. The extracted NAT information includes the internal address of the internal host, the external address of the internal host, and the external address of the external host, which are the same information elements as those stored in an entry in the address translation table  17 . The extracted NAT information in the illustrated example includes “A”, “a 1 ”, and “c”, as the internal address of the internal host, the external address of the internal host, and the external address of the external host, respectively. 
     The NAT information extraction unit  28  generates a packet used for transmitting the extracted NAT information to the post-change address translation device  3  specified by the movement information, and outputs the created packet to the transmission and reception unit  24 . The transmission and reception unit  24  transmits the extracted NAT information to the post-change address translation device  3  through the internal network  2 . Further, the extracted NAT information may be transmitted through the external network  90 . In this case, the NAT information extraction unit  28  outputs, to the transmission and reception unit  21 , the packet storing therein the extracted NAT information. 
     Upon receiving extracted NAT information from another address translation device  3 , the NAT information addition unit  29  adds, to the address translation table  17 , the entry included in the received extracted NAT information. As a result, when the internal host  4  has moved, the same entry as an entry that is stored in the address translation table  17  in the pre-change address translation device  3  and includes the address of the internal host  4  is added to the address translation table  17  in the post-change address translation device  3 . 
     Consequently, when a packet transmitted by the internal host  4  that has moved reaches the post-change address translation device  3  and the internal address translation unit  26  translates the transmission source address of the packet, the transmission source address is translated into the external address assigned by the pre-change address translation device  3 . The reason thereof will be described hereinafter. 
     Now, it is assumed that the first internal host  4   a  has moved, and hence an address translation device that is to perform address translation for a packet transmitted from the first internal host  4   a  changes from the first address translation device  3   a  to the second address translation device  3   b.    
       FIG. 8  is a schematic diagram illustrating an example of a packet flow when an internal host has moved, according to a first embodiment.  FIG. 8  illustrates an example where a movement of the first internal host  4   a  has occurred. In the case, the first address translation device  3   a  corresponds to the pre-change address translation device, and the second address translation device  3   b  corresponds to the post-change address translation device. 
     A communication path through which a packet is transferred from the external host  91  to the first internal host  4   a  is indicated by a reference numeral  100 . As illustrated by an arrow  101 , the external host  91  transmits a packet having the transmission destination address “a 1 ” and the transmission source address “c”. The transmission destination address “a 1 ” is an external address assigned to the first internal host  4   a  by the first address translation device  3   a , and the transmission source address “c” is the external address of the external host  91 . 
     Firstly, the first address translation device  3   a  receives a packet having the transmission destination address “a 1 ” and the transmission source address “c” before the first internal host  4   a  moves. In this case, the identifier designation unit  27  in the first address translation device  3   a  adds, to the address translation table  17 , a new entry including the internal address “A” and the external address “a 1 ” of the first internal host  4   a  and the external address “c” of the external host  91 . 
       FIG. 9A  is a diagram illustrating an example of an address translation table, according to a first embodiment.  FIG. 9A  illustrates an example where the above mentioned new entry is added to the address translation table  17  of the first address translation device  3   a.    
     In addition, before the first internal host  4   a  moves, the second address translation device  3   b  has received, from another external host having an external address “x”, a packet addressed to the second internal host  4   b . As a result, in the address translation table  17  of the second address translation device  3   b , an entry exists that includes the internal address “B” and the external address “b 2 ” of the second internal host  4   b  and the external address “x” of the external host. 
       FIG. 9B  is a diagram illustrating an example of an address translation table, according to a first embodiment.  FIG. 9B  illustrates an example where the above mentioned entry is stored in the address translation table  17  in the second address translation device  3   b  before the first internal host  4   a  moves. 
     Next, the first internal host  4   a  moves, and hence the first address translation device  3   a  receives movement information including information on the first internal host  4   a  that has moved. In an embodiment, the movement information may be transmitted to the first address translation device  3   a  by the host management server  5 . 
     For example, the host management server  5  performs the following operations (1) to (5) when the first internal host  4   a  moves. 
     (1) The host management server  5  updates the content of an information element “address translation device” of the entry that is stored in the internal host management table  6  in association with the first internal host  4   a.    
     (2) The host management server  5  transmits, to the first internal host  4   a , a movement instruction for causing the first internal host  4   a  to perform processing for movement thereof. 
     (3) The host management server  5  instructs the first internal host  4   a  to change a default gateway from the first address translation device  3   a  to the second address translation device  3   b.    
     (4) The host management server  5  transmits, to the DNS server  92 , an address change instruction for updating the DNS record  93  of the first internal host  4   a.    
     (5) The host management server  5  transmits movement information to the first address translation device  3   a.    
     The NAT information extraction unit  28  of the first address translation device  3   a  creates extracted NAT information storing therein an entry including the internal address “A” and the external address “a 1 ” of the first internal host  4   a  and the external address “c” of the external host  91 . The first address translation device  3   a  transmits the created extracted NAT information to the second address translation device  3   b . Upon receiving the extracted NAT information, the NAT information addition unit  29  of the second address translation device  3   b  adds, to the address translation table  17 , the same entry as an entry stored in the received extracted NAT information. 
       FIG. 9C  is a diagram illustrating an example of an address translation table, according to a first embodiment.  FIG. 9C  illustrates an example where the entry of the first internal host  4   a  is added to the address translation table  17  of the second address translation device  3   b.    
       FIG. 10  is a diagram illustrating an example of a sequence of communication operations, according to a first embodiment.  FIG. 10  illustrates operations in association with the transmission timings of pieces of information including the movement instruction, the address of the address translation device, the address change instruction, the movement information, and the extracted NAT information, which have been described above. 
     In operation AA, when the first internal host  4   a  has moved, the host management server  5  transmits, to the first internal host  4   a , the movement instruction and the address of the second address translation device  3   b  serving as the post-change address translation device. 
     In operation AB, the host management server  5  transmits an address change instruction to the DNS server  92 . 
     In operation AC, the host management server  5  transmits the movement information to the first address translation device  3   a  serving as the pre-change address translation device. 
     In operation AD, the first address translation device  3   a  transmits the extracted NAT information to the second address translation device  3   b.    
     Here,  FIG. 8  is referred to. When the external host  91  is not yet notified of the update of the DNS record  93  due to the movement of the first internal host  4   a , the external host  91  transmits a packet having the transmission destination address “a 1 ” in the similar way as a packet designated by the arrow  101  in  FIG. 8 . The packet having the transmission destination address “a 1 ” reaches the first address translation device  3   a , and the transmission destination address “a 1 ” is translated into the internal address “A” by the first address translation device  3   a  as indicated by an arrow  102 . After that, this packet is transmitted within the internal network  2  and reaches the first internal host  4   a.    
     A reference numeral  110  indicates a transmission path through which a response packet is sent back to the external host  91  from the first internal host  4   a  after movement, in the same sequence of communication operations in which the packet designated by the arrow  101  has been transmitted. Here, the first internal host  4   a  creates a response packet by swapping the transmission source address “c” and the transmission destination address “A” of the received packet designated by the arrow  102 , and sends back the created response packet to the external host  91 . An arrow  111  indicates the created response packet. 
     The response packet transmitted by the first internal host  4   a  reaches the second address translation device  3   b . In accordance with the address translation table  17 , the internal address translation unit  26  of the second address translation device  3   b  translates the internal address of the inputted response packet into an external address. As illustrated in  FIG. 9C , in the address translation table  17 , an entry exists that matches the combination of the internal address “A” of the first internal host  4   a  serving as the transmission source address of the response packet and the external address “c” of the external host  91  serving as the transmission destination address of the response packet. 
     Therefore, the internal address translation unit  26  translates the transmission source address “A” of the response packet into the external address “a 1 ” of the first internal host  4   a  that is being stored in the matched entry. A packet after translation has the transmission destination address “c” and the transmission source address “a 1 ” both of which serve as external addresses, as illustrated by an arrow  112  in  FIG. 8 . In such a way as described above, the transmission source address of the received response packet is translated, by the second address translation device  3   b , into the external address “a 1 ” that has been assigned to the first internal host  4   a  by the first address translation device  3   a.    
     The transmission source address “a 1 ” of the response packet received by the external host  91  (as designated by the arrow  112 ) matches the transmission destination address “a 1 ” of the packet transmitted by the external host  91  (as designated by the arrow  101 ). As a result, the external host  91  is able to identify the transmitted packet and the response packet as packets transmitted and received in the same sequence of communication operations, it is possible to continue communication. 
     The above-mentioned processing of the address translation device will be described using a flowchart. 
       FIG. 11  is a diagram illustrating an example of an operational flowchart for a first address translation device, according to a first embodiment.  FIG. 11  illustrates an example of processing performed by the first address translation device  3   a  when the first address translation device  3   a  operates as the pre-change address translation device. 
     In operation BA, the first address translation device  3   a  receives a packet from the external network  90 . 
     In operation BB, the first address translation device  3   a  checks the address of the received packet. 
     In operation BC, the first address translation device  3   a  determines whether or not, in the address translation table  17 , an entry exists that corresponds to the transmission source address and the transmission destination address of the received packet. When, in the address translation table  17 , the entry exits that corresponds to the addresses of the received packet (YES in operation BC), the processing proceeds to operation BE. When, in the address translation table  17 , the entry does not exit that corresponds to the addresses of the received packet (NO in operation BC), the processing proceeds to operation BD. 
     In operation BD, the identifier designation unit  27  of the first address translation device  3   a  adds, to the address translation table  17 , a new entry storing therein the external address and the internal address of the internal host  4  serving as a transmission destination and the external address of the external host  91 . After that, the processing proceeds to operation BE. 
     In operation BE, in accordance with the address translation table  17 , the external address translation unit  23  of the first address translation device  3   a  translates, into an internal address, the external address used as the transmission destination address of the received packet. 
     In operation BF, the transmission and reception unit  24  of the first address translation device  3   a  transfers the packet to the internal network  2 . 
     In operation BG, the NAT information extraction unit  28  of the first address translation device  3   a  determines whether or not the movement of the internal host  4  exists that is to be subjected to address translation by the first address translation device  3   a . The NAT information extraction unit  28  may be configured to determine whether or not the movement of the internal host  4  exists by determining whether or not movement information is received. When the movement of the internal host  4  exists (YES in operation BG), the processing proceeds to operation BH. When the movement of the internal host  4  does not exist (NO in operation BG), the processing is terminated. 
     In operation BH, the NAT information extraction unit  28  extracts an entry including the address of the internal host  4  to move, from the address translation table  17 , and transmits, to the post-change address translation device, the extracted NAT information storing therein the extracted entry. After that, the processing is terminated. 
       FIG. 12  is a diagram illustrating an example of an operational flowchart for a second address translation device, according to a first embodiment.  FIG. 12  illustrates processing performed in the second address translation device  3   b  when the second address translation device  3   b  operates as the post-change address translation device. 
     In operation CA, the NAT information addition unit  29  of the second address translation device  3   b  determines whether or not extracted NAT information has been received from another address translation device  3 . When the extracted NAT information has been received (YES in operation CA), the processing proceeds to operation CB, and otherwise (NO in operation CA), the processing proceeds to operation CC. 
     In operation CB, the NAT information addition unit  29  adds, to the address translation table  17 , the same entry as an entry included in the received extracted NAT information. After that, the processing proceeds to operation CC. 
     In operation CC, the second address translation device  3   b  receives a packet from the internal network  2 . 
     In operation CD, in accordance with the address translation table  17 , the internal address translation unit  26  translates, to an external address, the internal address of the received packet. When the entry of the first internal host  4   a  that was created by the first address translation device  3   a  has been added, in operation CB, to the address translation table  17  based on the received extracted NAT information, the transmission source address of the received packet from the first internal host  4   a  is translated into an external address assigned by the first address translation device  3   a.    
     In operation CE, the transmission and reception unit  21  transfers the packet to the external network  90 . After that, the processing is terminated. 
     As mentioned above, when the internal host  4  moves, a case occurs where a packet transmitted from the external host  91  to the internal host  4  and a packet transmitted from the internal host  4  to the external host  91  transit through the different address translation devices  3 . According to the embodiment, in such a case, it is possible to match the external addresses of the internal hosts  4  that are specified by a packet transmitted from the external host  91  to the internal host  4  and by a response packet transmitted from the internal host  4  to the external host  91 , with each other. As a result, the external host  91  becomes able to identify the transmitted packet and the response packet as packets transmitted and received in the same sequence of communication operations, thereby allowing the external host  91  to continue communication. 
     &lt;4. Second Embodiment of Address Translation Device&gt; 
     Next, another embodiment of the address translation device  3  will be described. In the embodiment, as an identifier for identifying a state in which a packet from the external host  91  has transited through the address translation device  3 , an internal address that is assigned to the external host  91  by the address translation device  3  is used. 
       FIG. 13  is a diagram illustrating an example of an address translation table, according to a second embodiment. The address translation table  17  includes, as information elements, “the internal address of an internal host”, “the external address of the internal host”, “the internal address of an external host”, and “the external address of the external host”. 
     The information elements “the internal address of the internal host”, “the external address of the internal host”, and “the internal address of the external host” are the same as those in the address translation table  17  illustrated in  FIG. 5 . The information element “the internal address of the external host” is an internal address that is assigned to the external host  91  by the address translation device  3 . The illustrated example indicates that the address translation device  3  has assigned the external address “a 1 ” to the first internal host  4   a  having the internal address “A” and routes, through the address translation device  3 , a packet whose transmission source is the external host  91  having the external address “c” and whose destination is the first internal host  4   a . Furthermore, the illustrated example indicates that the address translation device  3  has assigned an internal address “PA 12 ” to the external host  91 . 
     When there exist no entries, in the address translation table  17 , that correspond to the addresses of a received packet, the identifier designation unit  27  acquires an unused internal address from the pool address table, and assigns the acquired internal address to the external host  91 . The pool address table indicates the list of addresses that are allocated to the address translation device  3  as internal addresses that are allowed to be assigned to the external host  91  by the address translation device  3 . The internal addresses stored in the pool address tables of the individual address translation devices  3   a  to  3   c  are specified so as not to overlap with each other. The pool address table may be stored in the auxiliary storage device  11 , as the data  16  illustrated in  FIG. 3 . 
       FIG. 14  is a diagram illustrating an example of a pool address table, according to a second embodiment. The pool address table includes information elements “address” and “usage flag”. The information element “address” indicates an internal address that is allowed to be assigned to the external host  91  by the address translation device  3 . The information element “usage flag” indicates whether or not each internal address has been assigned to the external host  91  by the address translation device  3 . The illustrated example indicates that internal addresses “PA 11 ” and “PA 12 ” are allocated to the address translation device  3 , as internal addresses allowed to be assigned to the external host  91  by the address translation device  3 . In addition, the internal address “PA 11 ” has already been used, and the internal address “PA 12 ” has not been used yet. 
     The identifier designation unit  27  creates an entry storing therein the external address and the internal address of the internal host  4  serving as a transmission destination, the external address of the external host  91 , and the internal address of the external host  91  that has been acquired from the pool address table, and adds the created entry to the address translation table  17 . 
       FIG. 15  is a schematic diagram illustrating an example of a packet flow when an internal host moves, according to a second embodiment.  FIG. 15  illustrates a packet flow when the first internal host  4   a  has moved. The usage of the reference numeral  100  and the arrow  101  is the same as those in  FIG. 8 . The first address translation device  3   a  receives, for the first time, a packet having the transmission destination address “a 1 ” and the transmission source address “c” before the first internal host  4   a  moves. In this case, the identifier designation unit  27  of the first address translation device  3   a  acquires, as the internal address for the external host  91 , the address “PA 12 ” associated with the information element “usage flag” having value “unused”, from the pool address table. The identifier designation unit  27  adds, to the address translation table  17 , a new entry including the internal address “A” and the external address “a 1 ” of the first internal host  4   a  and the internal address “PA 12 ” and the external address “c” of the external host  91 . 
       FIG. 16A  is a diagram illustrating an example of an address translation table, according to a second embodiment.  FIG. 16A  illustrates the address translation table  17  of the first address translation device  3   a  to which the above mentioned new entry is added. 
     In addition, before the movement of the first internal host  4   a , the second address translation device  3   b  has received, from another external host having an external address “x”, a packet addressed to the second internal host  4   b . Therefore, there exists, in the address translation table  17  of the second address translation device  3   b , an entry that includes the internal address “B” and the external address “b 2 ” of the second internal host  4   b  and the internal address “PA 21 ” and the external address “x” of the external host. 
       FIG. 16B  is a diagram illustrating an example of an address translation table, according to a second embodiment.  FIG. 16B  illustrates the address translation table  17  of the second address translation device  3   b  before the first internal host  4   a  moves. 
     When the first internal host  4   a  moves, the first address translation device  3   a  receives movement information including information on the first internal host  4   a  that has moved. The NAT information extraction unit  28  of the first address translation device  3   a  creates extracted NAT information storing therein an entry that stores the internal address “A” and the external address “a 1 ” of the first internal host  4   a  and the internal address “PA 12 ” and the external address “c” of the external host  91 . 
       FIG. 17  is a diagram illustrating an example of extracted NAT information, according to a second embodiment. The first address translation device  3   a  transmits the extracted NAT information to the second address translation device  3   b.    
     Upon receiving the extracted NAT information, the NAT information addition unit  29  of the second address translation device  3   b  adds, to the address translation table  17 , the same entry as an entry stored in the received extracted NAT information. 
       FIG. 16C  is a diagram illustrating an example of an address translation table, according to a second embodiment.  FIG. 16C  illustrates the address translation table  17  of the second address translation device  3   b  to which the above mentioned entry is added. 
     When the external host  91  transmits a packet having the transmission destination address “a 1 ” in the same way as the packet designated by the arrow  101  in  FIG. 15 , this packet reaches the first address translation device  3   a . In accordance with the address translation table  17 , the external address translation unit  23  translates, into internal addresses, the external addresses of the first internal host  4   a  and the external host  91  stored in the received packet. As a result, the transmission destination address “a 1 ” and the transmission source address “c” in the received packet are translated into the internal addresses “A” and “PA 12 ”, respectively. The packet after translation is indicated by an arrow  102  in  FIG. 15 . This packet is transmitted within the internal network  2  and reaches the first internal host  4   a.    
     A reference numeral  110  indicates a transmission path through which a response packet is sent back from the first internal host  4   a  after movement, to the external host  91  in the same sequence of communication operations in which the packet indicated by the arrow  101  has been transmitted. The first internal host  4   a  creates a response packet to be sent back to the external host  91 , by swapping the transmission source address “PA 12 ” and the transmission destination address “A” of the received packet designated by the arrow  102 . An arrow  111  indicates the created response packet. 
     The created response packet reaches the second address translation device  3   b . In accordance with the address translation table  17 , the internal address translation unit  26  of the second address translation device  3   b  translates the internal address of the received response packet into an external address. In the address translation table  17 , there exists an entry that matches the combination of the internal address “A” of the first internal host  4   a  serving as the transmission source address of the response packet and the internal address “PA 12 ” of the external host  91  serving as the transmission destination address of the response packet. Therefore, the internal address translation unit  26  translates the transmission source address “A” and the transmission destination address “PA 12 ” of the response packet into the external addresses “a 1 ” and “c”, respectively. The response packet after translation is illustrated by an arrow  112 . That is, a transmission source address after translation becomes the external address “a 1 ” that was assigned to the first internal host  4   a  by the first address translation device  3   a.    
     In this way, the transmission source address “a 1 ” of the response packet received by the external host  91  matches the transmission destination address “a 1 ” of the packet transmitted from the external host  91 . As a result, the external host  91  becomes able to identify the transmitted packet and the received response packet as packets transmitted and received in the same sequence of communication operations, thereby allowing the external host  91  to continue communication. 
       FIG. 18  is a diagram illustrating an example of an operational flowchart performed by a first address translation device, according to a second embodiment. 
     Processing for operations DA to DC is the same as the processing for operations BA to BC in  FIG. 11 . 
     When there exists, in the address translation table  17 , an entry that corresponds to the addresses of a received packet (YES in operation DC), the processing proceeds to operation DF, and otherwise (NO in operation DC), the processing proceeds to operation DD. 
     In operation DD, the identifier designation unit  27  of the first address translation device acquires an internal address for the external host  91  from the pool address table. 
     In operation DE, the identifier designation unit  27  adds, to the address translation table  17 , a new entry storing therein the external address and the internal address of the internal host  4  serving as a transmission destination and the internal address and the external address of the external host  91 . After that, the processing proceeds to operation DF. 
     Processing for operations DF to DI is the same as the processing for operations BE to BH in  FIG. 11 . 
       FIG. 19  is a schematic diagram illustrating an example of a packet flow when an internal host moves, according to a second embodiment.  FIG. 19  illustrates a packet flow that is performed after the external host  91  has been notified of the DNS record updated owing to the movement of the first internal host  4   a . The usage of the reference numerals  100  to  102  and  110  to  112  is the same as in  FIG. 15 . 
     After the external host  91  has been notified of the DNS record, a packet that is to be transmitted from the external host  91  to the first internal host  4   a  is transmitted along a communication path indicated by a reference numeral  120 . As indicated by an arrow  121 , the external host  91  transmits a packet having a transmission destination address “a 2 ” and the transmission source address “c”. The transmission destination address “a 2 ” is an external address that was assigned to the first internal host  4   a  by the second address translation device  3   b.    
     When the second address translation device  3   b  receives the packet having the transmission destination address “a 2 ” and the transmission source address “c”, for the first time, the second address translation device  3   b  assigns an internal address to the external host  91 . In this example, an internal address “PA 21 ” is assigned to the external host  91  by the second address translation device  3   b . The second address translation device  3   b  translates the transmission destination address and the transmission source address of the received packet into the internal addresses “A” and “PA 21 ”, respectively. The packet after translation is indicated by an arrow  122  in  FIG. 19 . 
     A reference numeral  130  indicates a transmission path through which the first internal host  4   a  that has received the packet indicated by the arrow  122  sends back a response packet to the external host  91  of a transmission source. The response packet indicated by an arrow  131  includes the transmission destination address “PA 21 ” and the transmission source address “A”. 
     In the same way as the transmission path indicated by the reference numeral  110 , the transmission path indicated by the reference symbol  130  is a transmission path through which the response packet is transmitted to the external network  90  via the second address translation device  3   b . That is, when a first packet that has transited via the first address translation device  3   a  has been received and when a second packet that has transited via the second address translation device  3   b  has been received, response packets corresponding to these received packets are transmitted to the external network  90  via the second address translation device  3   b.    
     The transmission destination address of a first response packet corresponding to the first packet received through the first address translation device  3   a  is the internal address “PA 12 ” assigned by the first address translation device  3   a , as indicated by the arrow  111 . The transmission destination address of a second response packet corresponding to the second packet received through the second address translation device  3   b  is the internal address “PA 21 ” assigned by the second address translation device  3   b  and different from the transmission destination address of the first response packet corresponding to the first packet received through the first address translation device  3   a . Therefore, it is possible for the second address translation device  3   b  to distinguish the first response packet corresponding to the first packet received through the first address translation device  3   a  from the second response packet corresponding to the second packet received through the second address translation device  3   b.    
     As a result, it is possible for the second address translation device  3   b  to translate the transmission source address of the first response packet corresponding to the first packet received through the first address translation device  3   a  into the external address “a 1 ” assigned by the first address translation device  3   a . In addition, it is possible for the second address translation device  3   b  to translate the transmission source address of the second response packet corresponding to the second packet received through the second address translation device  3   b  into the external address “a 2 ” assigned by the second address translation device  3   b.    
     According to the embodiment, even if response packets corresponding to the received packets that have transited through the first address translation device  3   a  and the second address translation device  3   b , respectively, are mixed with each other after the external host  91  has been notified of the update of the DNS record, it is possible to perform address translation for these response packets by distinguishing these response packets from each other. 
     &lt;5. Third Embodiment of Address Translation Device&gt; 
     Next, another embodiment of the address translation device  3  will be described. In this embodiment, as an identifier for identifying a state in which a packet from the external host  91  has transited through the address translation device  3 , the external IP address and the external port number of the external host  91  are used. In the following description and attached drawings, the port number will be also expressed simply as a “port” for convenience. 
       FIG. 20  is a diagram illustrating an example of an address translation table, according to a third embodiment. The address translation table  17  includes, as information elements, “the internal IP address of an internal host”, “the external IP address of the internal host”, “the external IP address of an external host”, and “the external port of the external host”. The information elements “the internal IP address of the internal host” and “the external IP address of the internal host” indicate the internal IP address and the external IP address of the internal host  4 , respectively. 
     The information element “the external IP address of the external host” indicates the external IP address of the external host  91  serving as the transmission source of a packet that has been transmitted to the internal host  4  through the address translation device  3 . The information element “the external port of the external host” indicates an external port used by an application in the external host  91  that has transmitted the packet. The illustrated example indicates that the address translation device  3  assigns an external IP address “a 1 ” to the first internal host  4   a  having an internal IP address “A”. In addition, the illustrated example indicates that the address translation device  3  has received a packet whose destination is the first internal host  4   a , from an application that utilizes an external port “ 90 ” in the external host  91  having an external IP address “c”. 
     When a packet has been received from the external network  90 , the identifier designation unit  27  determines whether or not there exists, in the address translation table  17 , an entry that corresponds to the transmission source IP address and the transmission source port of this packet and the external IP address of the internal host  4  serving as the transmission destination of this packet. When the corresponding entry does not exist in the address translation table  17 , the identifier designation unit  27  creates a new entry storing therein the external IP address and the internal IP address of the internal host  4  serving as a transmission destination and the external IP address and the external port of the external host  91 , and adds the created new entry to the address translation table  17 . 
       FIG. 21  is a schematic diagram illustrating an example of a packet flow when an internal host moves, according to a third embodiment. Hereinafter, with reference to  FIG. 21 , processing will be described that is performed in the embodiment when the first internal host  4   a  has moved. The usage of the reference numeral  100  is the same as in  FIG. 8 . As indicated by an arrow  101 , the external host  91  transmits a packet having a transmission destination IP address “a 1 ”, a transmission destination port “ 80 ”, the transmission source IP address “c”, and the transmission source IP port “ 90 ”. The transmission destination IP address “a 1 ” is an external address assigned to the first internal host  4   a  by the first address translation device  3   a , and the transmission destination port “ 80 ” is one of ports of the first internal host  4   a  that is to be the transmission destination of the packet. The transmission source IP address “c” is the external IP address of the external host  91 , and the transmission source port “ 90 ” is the external port of the external host  91 . 
     The first address translation device  3   a  receives a packet indicated by the arrow  101 , for the first time, before the movement of the first internal host  4   a . The identifier designation unit  27  of the first address translation device  3   a  adds, to the address translation table  17 , a new entry storing the internal IP address “A” and the external IP address “a 1 ” of the first internal host  4   a  and the external IP address “c” and the external port “ 90 ” of the external host  91 . 
       FIG. 22A  is a diagram illustrating an example of an address translation table, according to a third embodiment.  FIG. 22A  illustrates the address translation table  17  of the first address translation device  3   a  to which the above mentioned new entry has been added. 
     In addition, before the movement of the first internal host  4   a , the second address translation device  3   b  has received, from another external host having an external address “x”, a packet addressed to the second internal host  4   b . Therefore, there exists, in the address translation table  17  of the second address translation device  3   b , an entry that includes the internal IP address “B” and the external IP address “b 2 ” of the second internal host  4   b  and the external IP address “x” and the external port “ 70 ” of the external host. 
       FIG. 22B  is a diagram illustrating an example of an address translation table, according to a third embodiment.  FIG. 22B  illustrates the address translation table  17  of the second address translation device  3   b  before the first internal host  4   a  moves. 
     The first address translation device  3   a  receives movement information owing to the movement of the first internal host  4   a . The NAT information extraction unit  28  of the first address translation device  3   a  creates extracted NAT information storing therein an entry that stores the internal IP address “A” and the external IP address “a 1 ” of the first internal host  4   a  and the external IP address “c” and the external port “ 90 ” of the external host  91 . 
       FIG. 23  is a diagram illustrating an example of extracted NAT information, according to a third embodiment. The first address translation device  3   a  transmits the created extracted NAT information to the second address translation device  3   b.    
     The NAT information addition unit  29  of the second address translation device  3   b  adds, to the address translation table  17 , an entry that is the same as an entry stored in the received extracted NAT information. 
       FIG. 22C  is a diagram illustrating an example of an address translation table, according to a third embodiment.  FIG. 22C  illustrates the address translation table  17  of the second address translation device  3   b  to which the above mentioned entry of the first internal host  4   a  is added. 
     When the external host  91  transmits a packet having the transmission destination address “a 1 ” in the same way as indicated by the arrow  101  in  FIG. 21 , this packet reaches the first address translation device  3   a . In accordance with the address translation table  17 , the external address translation unit  23  translates, into an internal address, the external address of the first internal host  4   a  stored in the received packet. As a result, the transmission destination address “a 1 ” stored in the received packet is translated into the internal address “A” as indicated by an arrow  102  in  FIG. 21 . This packet is transmitted within the internal network  2  and reaches the first internal host  4   a.    
     A reference numeral  110  indicates a transmission path through which a response packet is sent back to the external host  91  from the first internal host  4   a  after movement, in the same sequence of communication operations in which the packet indicated by the arrow  101  has been transmitted. The first internal host  4   a  creates a response packet to be sent back to the external host  91 , by swapping the transmission source address “c” and the transmission destination address “A” of the received packet of the arrow  102 . An arrow  111  indicates the created response packet. 
     The response packet reaches the second address translation device  3   b . In accordance with the address translation table  17 , the internal address translation unit  26  of the second address translation device  3   b  translates the internal address of the received response packet into an external address. In the address translation table  17 , there exists an entry that matches the combination of the internal address “A” of the first internal host  4   a  serving as the transmission source address of the response packet and the external address “c” of the external host  91  serving as the transmission destination address thereof. Therefore, the internal address translation unit  26  translates the transmission source address “A” of the response packet into the external address “a 1 ”. The response packet after translation is designated by an arrow  112 , and a transmission source address after translation becomes the external address “a 1 ” that was assigned to the first internal host  4   a  by the first address translation device  3   a.    
     The transmission source address “a 1 ” of the response packet received by the external host  91  matches the transmission destination address “a 1 ” of the packet transmitted by the external host  91 . As a result, the external host  91  is able to identify the transmitted packet and the response packet as packets transmitted and received in the same sequence of communication operations, thereby allowing the external host  91  to continue communication. 
     Whether or not information on the update of the DNS record reaches the external host  91  depends on each application operating in the same external host  91 . For example, a packet transmitted from a first application that has not received the update of the DNS record reaches the internal host  4  through the pre-change address translation device, whereas a packet transmitted from a second application that has received the update of the DNS record reaches the internal host  4  through the post-change address translation device. In this case, a state occurs where even packets transmitted from the same external host  91  reach the internal host  4  through different address translation devices, depending on applications that have processed the packets. 
     According to the embodiment, the address translation table has a different entry with respect to each port of the external host  91 . Accordingly, even if response packets corresponding to packets received through address translation devices  3  different depending on applications are mixed with each other, it is possible to perform address translation for these response packets while distinguishing these response packets from each other. 
     &lt;6. Fourth Embodiment of Address Translation Device&gt; 
     Next, another embodiment of the address translation device  3  will be described. In this embodiment, as an identifier identifying a state indicating that a packet from the external host  91  has transited through the address translation device  3 , an internal IP address and an internal port that are assigned to the external host  91  by the address translation device  3  are used. 
       FIG. 24  is a diagram illustrating an example of an address translation table, according to a fourth embodiment. As depicted in  FIG. 24 , the address translation table  17  includes, as information elements, “the internal IP address of an internal host”, “the external IP address of the internal host”, “the internal IP address of an external host”, “the internal port of the external host”, “the external IP address of the external host”, and “the external port of the external host”. The information elements “the internal IP address of the internal host”, “the external IP address of the internal host”, “the external IP address of the external host”, and “the external port of the external host” are the same as those in the address translation table  17  illustrated in  FIG. 20 . 
     The information element “the internal IP address of the external host” is an internal IP address that is assigned to the external host  91  by the address translation device  3 . In addition, the information element “the internal port of the external host” is an internal port that is assigned, by the address translation device  3 , to a port used by an application in the external host  91 . 
     In the illustrated example, the address translation device  3  has assigned the external IP address “a 1 ” to the first internal host  4   a  having the internal IP address “A”. In addition, the illustrated example indicates that a packet whose destination is the first internal host  4   a  has been received from an application utilizing an external port “ 90 ” of the external host  91  having the external IP address “c”. In addition, in the illustrated example, the address translation device  3  has assigned the internal IP address “PA 12 ” and an internal port “p 2 ” to the external host  91  and an application of the external host  91 , respectively. 
     When there exist, in the address translation table  17 , no entries that correspond to the addresses of a received packet, the identifier designation unit  27  acquires an unused IP internal address and an unused internal port from the pool address table, and assigns the acquired IP internal address and internal port to the external host  91 . 
       FIG. 25  is a diagram illustrating an example of a pool address table, according to a fourth embodiment. The pool address table includes information elements “IP address”, “port”, and “usage flag”. The combination of the information elements “IP address” and “port” indicates the combination of an internal IP address and a port that are allowed, by the address translation device  3 , to be assigned to the external host  91  and the application thereof. The usage of the information element “usage flag” is the same as the example in  FIG. 14 . 
     In this embodiment, the internal address assigned to the external host by the address translation device allows response packets corresponding to the received packets that have transited through different address translation devices, to be distinguished from each other. Therefore, according to the fourth embodiment, even if response packets corresponding to the received packets that have transited through different address translation devices are mixed with each other after the update of the DNS record has reached the external host  91 , address translation may be performed so that these response packets are distinguished from each other. 
     In the fourth embodiment, even if packets transmitted from different applications have transited through different address translation devices, response packets corresponding to these packets are distinguished from each other based on a port used by an application in the external host. Accordingly, even if response packets corresponding to the received packets that have transited through different address translation devices depending on applications are mixed with each other, address translation may be performed so that these response packets are distinguished from each other. 
     &lt;7. Fifth Embodiment of Address Translation Device&gt; 
     Next, another embodiment of the address translation device  3  will be described. In this embodiment, when the internal host  4  has moved, the pre-change address translation device  3  memorizes information on the internal host  4 . When the pre-change address translation device  3  has added an entry for the internal host  4  to the address translation table  17  after the movement of the internal host  4 , the pre-change address translation device  3  transmits, to the post-change address translation device  3 , extracted NAT information storing therein the added entry. The post-change address translation device  3  adds, to the address translation table  17 , the entry stored in the received extracted NAT information. 
       FIG. 26  is a diagram illustrating a configuration example of an address translation device, according to a fifth embodiment. The address translation device  3  includes a mobile host information generation unit  31  and a mobile host table  30 . In accordance with the address translation program  15  stored in the auxiliary storage device  11 , the processor  10  in  FIG. 2  executes information processing due to the mobile host information generation unit  31 . In addition, as the data  16 , the mobile host table  30  is stored in the auxiliary storage device  11 . 
     In addition, in the following description, a case will be described where the address translation device  3  according to the above-mentioned fifth embodiment includes the mobile host information generation unit  31  and the mobile host table  30 . However, the address translation device  3  according to the first embodiment, the second embodiment, the third embodiment, or the fourth embodiment, which were described above, may also be configured to include the mobile host information generation unit  31  and the mobile host table  30 . 
     Upon receiving movement information indicating the movement of an internal host  4  that has transmitted packets whose address translation has been performed by the address translation device  3 , the mobile host information generation unit  31  of the address translation device  3  adds, to the mobile host table  30 , an entry relating to the internal host  4 , based on the received movement information. 
       FIG. 27  is a diagram illustrating an example of a mobile host table, according to a fifth embodiment. 
     As depicted in  FIG. 27 , the mobile host table  30  includes information elements “the internal address of an internal host”, “the external address of the internal host”, and “an address translation device”. The information elements “the internal address of the internal host” and “the external address of the internal host” indicate the internal address and the external address of the internal host that has moved, respectively. The information element “the address translation device” indicates the address of the post-change address translation device  3 . The illustrated example indicates that the first internal host  4   a  having the internal address “A” and the external address “a 1 ” has moved and the post-change address translation device  3  is the second address translation device  3   b.    
     When a new entry is added to the address translation table  17 , the NAT information extraction unit  28  determines whether or not there exists, in the mobile host table  30 , an entry that relates to the same internal host  4  for which the new entry is added to the address translation table  17 . Namely, the NAT information extraction unit  28  determines whether or not an internal host  4  for which a new entry is added to the address translation table  17  is the internal host  4  that has moved. 
     When the internal host  4  for which a new entry is added to the address translation table  17  is the internal host  4  that has moved, the NAT information extraction unit  28  creates extracted NAT information storing therein the added new entry, and transmits the created extracted NAT information to the post-change address translation device  3 . The NAT information addition unit  29  of the post-change address translation device  3  adds the received entry to the address translation table  17  thereof. 
       FIG. 28  is a schematic diagram illustrating an example of a packet flow, according to a fifth embodiment.  FIG. 28  illustrates a packet flow that is performed when, after the movement of the first internal host  4   a , an entry for the first internal host  4   a  is added to the address translation table  17  of the first address translation device  3   a  serving as the pre-change address translation device. 
     Now, a case will be considered where the update of the DNS record  93  due to the movement of the first internal host  4   a  has not reached an external host  95  having an external address “d”. This external host  95  is an external host that has not established communication with the first internal host  4   a  yet before the update of the DNS record  93 . When the first internal host  4   a  has moved, an entry for the first internal host  4   a  is added to mobile host table  30  of the first address translation device  3   a.    
     A reference numeral  140  indicates a communication path through which a packet is transmitted from the external host  95  to the first internal host  4   a . As illustrated by an arrow  141 , the external host  95  transmits a packet having the transmission destination address “a 1 ” and the transmission source address “d”. 
     At this time, the first address translation device  3   a  receives the packet having the transmission destination address “a 1 ” and the transmission source address “d” for the first time. In this case, the identifier designation unit  27  of the first address translation device  3   a  adds, to the address translation table  17 , a new entry storing the internal address “A” and the external address “a 1 ” of the first internal host  4   a  and the internal address “PA 13 ” and the external address “d” of the external host  91 . 
       FIG. 29A  is a diagram illustrating an example of an address table, according to a fifth embodiment.  FIG. 29A  illustrates the state of the address translation table  17  of the first address translation device  3   a  to which the above mentioned new entry is added. 
       FIG. 29B  is a diagram illustrating an example of an address table, according to a fifth embodiment.  FIG. 29B  illustrates the state of the address translation table  17  of the second address translation device  3   b  before the first address translation device  3   a  receives the packet indicated by the arrow  141 . An entry at the first row in the address translation table  17  is an entry added by the second address translation device  3   b  when the second address translation device  3   b  has received, from the external network  90 , a packet addressed to the second internal host  4   b . An entry at the second row in the address translation table  17  is an entry that is added, by the second address translation device  3   b , upon receiving extracted NAT information from the first address translation device  3   a  when the first internal host  4   c  has moved. 
     When an entry for the first internal host  4   a  has been added to the address translation table  17 , the NAT information extraction unit  28  of the first address translation device  3   a  determines whether or not there exists an entry for the first internal host  4   a  in the mobile host table  30 . In this case, since the entry for the first internal host  4   a  exists in the mobile host table  30 , the NAT information extraction unit  28  transmits, to the second address translation device  3   b , extracted NAT information storing therein the added entry. The second address translation device  3   b  adds the entry included in the received extracted NAT information to the address translation table  17 . As a result, the state of the address translation table  17  of the second address translation device  3   b  shift to a state as illustrated in  FIG. 29C . 
     The packet indicated by the arrow  141  is subjected to address translation by the external address translation unit  23  of the first address translation device  3   a . In accordance with the address translation table  17 , the external address translation unit  23  translates, into internal addresses, the external addresses of the first internal host  4   a  and the external host  91  stored in the received packet. As a result, the transmission destination address “a 1 ” and the transmission source address “d” of the received packet are translated into the internal addresses “A” and “PA 13 ”, respectively. A packet after translation is indicated by an arrow  142 . This packet is transmitted within the internal network  2  and reaches the first internal host  4   a.    
     A reference numeral  150  indicates a transmission path of a response packet that is sent back, to the external host  95 , from the first internal host  4   a  after movement, in the same sequence of communication operations in which the packet indicated by the arrow  141  has been transmitted. The first internal host  4   a  creates a response packet to be sent back to the external host  95 , by swapping the transmission source address “PA 13 ” and the transmission destination address “A” of the received packet indicated by an arrow  142 . An arrow  151  indicates the created response packet. 
     The response packet reaches the second address translation device  3   b . In accordance with the address translation table  17 , the internal address translation unit  26  of the second address translation device  3   b  translates the internal address of the received response packet to an external address. In the address translation table  17 , there exists an entry that matches the combination of the internal address “A” of the first internal host  4   a  serving as the transmission source address of the response packet and the internal address “PA 13 ” of the external host  95  serving as the transmission destination address of the response packet. Therefore, the internal address translation unit  26  translates the transmission source address “A” and the transmission destination address “PA 13 ” of the response packet into the external addresses “a 1 ” and “d”, respectively. The response packet after translation is indicated by an arrow  152 . In this way, the transmission source address after translation becomes the external address “a 1 ” assigned to the first internal host  4   a  by the first address translation device  3   a.    
     As described above, in some cases, after the movement of the internal host  4 , an entry for the internal host  4  is added to the address translation table  17  of the pre-change address translation device  3 . According to the fifth embodiment, it is possible to add, to the address translation table  17  of the post-change address translation device  3 , the same entry as an entry added to the address translation table  17  of the pre-change address translation device  3  after the movement of the internal host  4 . This allows the internal host  4  to continue communication with the external host  95  that has not established communication with the internal host  4  before the update of the DNS record  93  and has not received the update of the DNS record  93  yet. 
     &lt;8. Sixth Embodiment of Address Translation Device&gt; 
     Next, another embodiment of the address translation device  3  will be described. In this embodiment, a deletion time is set in an entry that is added to the address translation table  17  upon receiving the extracted NAT information. 
       FIG. 30  is a diagram illustrating an example of an address translation table, according to a sixth embodiment. The address translation table  17  includes, as information elements, “the internal address of an internal host”, “the external address of the internal host”, “the internal address of an external host”, “the external address of the external host”, and “a deletion time”. 
     The information elements “the internal address of the internal host”, “the external address of the internal host”, “the internal address of the external host”, and “the external address of the external host” are the same as those in the address translation table  17  illustrated in  FIG. 13 . The information element “the deletion time” specifies a time when this entry is to be deleted. The example depicted in  FIG. 30  indicates that an entry in which the information elements “the internal address of the internal host”, “the external address of the internal host”, “the internal address of the external host”, and “the external address of the external host” are set at “A”, “a 1 ”, “PA 12 ”, and “c”, respectively, is to be deleted at 9:21 a.m. on Mar. 31, 2011. 
     In the following description, a case will be described where a deletion time is set in an entry of the address translation table  17  of the address translation device  3  according to the above-mentioned fifth embodiment. However, the similar deletion time may be set in the first embodiment, the second embodiment, the third embodiment, or the fourth embodiment that were described above. 
     When the internal host  4  has moved, the host management server  5  specifies, using movement information, the time to live (TTL) of the entry that is added to the address translation table  17  upon receiving extracted NAT information. 
       FIG. 31  is a diagram illustrating an example of movement information, according to sixth embodiment. The movement information includes the identifier of the internal host  4  to move, the internal address of the internal host  4  to move, the address of the post-change address translation device  3 , and TTL specification. 
     The NAT information extraction unit  28  of the pre-change address translation device  3  creates extracted NAT information so that the extracted NAT information includes the specified TTL specification. The NAT information addition unit  29  of the post-change address translation device  3  determines the deletion time by adding a value of the TTL to a current time. The NAT information addition unit  29  sets the determined deletion time in the entry that is added to the address translation table  17  upon receiving the extracted NAT information. Alternatively, the NAT information extraction unit  28  may determine the deletion time, and creates extracted NAT information so that the created extracted NAT information includes the determined deletion time. 
     When the transmission of the update of the DNS record has completed, the entry that was added to the address translation table  17  upon receiving the extracted NAT information becomes unnecessary. According to this embodiment, since a time to live is set to the entry that was added to the address translation table  17  upon receiving the extracted NAT information, it is possible to delete unnecessary entries from the address translation table  17 , thereby avoiding the enlargement of the address translation table  17 . 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.