Patent Publication Number: US-2019173837-A1

Title: Communication device and communication system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a communication device and a communication system. 
     BACKGROUND ART 
     DHCP (Dynamic Host Configuration Protocol) server devices are used to simplify the setting of IP (Internet Protocol) addresses for clients connected to networks. 
     Moreover, a technique is disclosed to prevent the occurrence of duplicate allocation of IP addresses in a system that allocates the IP addresses to clients by using multiple DHCP server devices (for example, see PTL 1). 
     An IP address allocation system described in PTL 1 is an IP address allocation system that allocates IP addresses to clients by using multiple DHCP server devices allocatable to a first network. Each of the DHCP server devices stores an IP address range from the address range of the overall network, that is, the overall range of the IP addresses of the first network. The stored IP address range is allocatable to clients and does not overlap with the IP address range of the other DHCP server devices. When an allocation request for an IP address is received from a client, an IP address included in the address range not overlapping with the IP address range of the other server devices is allocated to the client with a specified lease period that is a period from the allocation to the release of the IP address. Moreover, when an extension request for the lease period of an existing IP address allocated from a client is received, permission to extend the lease period is transmitted to the client regardless of whether or not the existing IP address is included in the address range not overlapping with the IP address range of the other server devices. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2012-249213 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     The present disclosure provides a communication device and a communication system, by which if a plurality of communication devices having a DHCP server function are provided on the same network, the absence of the communication device having started a DHCP server on the network is prevented. 
     Solution to Problem 
     A communication device according to the present disclosure is a communication device in a communication system including a plurality of communication devices on a network, the plurality of communication devices each having a Dynamic Host Configuration Protocol (DHCP) server function of allocating an Internet Protocol (IP) address, the communication device including: a communicator that transmits a confirmation signal for confirming presence or absence of an other communication device having the DHCP server function started on the network; and a controller that controls the DHCP server function, in which the controller: transmits a first confirmation signal to the other communication device through the communicator; starts the DHCP server function and transmits a second confirmation signal to the other communication device through the communicator when a first response signal is not received from the other communication device in response to the first confirmation signal; stops the DHCP server function when a second response signal is received in response to the second confirmation signal; and performs a transmission control of a third confirmation signal to the other communication device through the communicator, based on the second response signal. 
     Furthermore, a communication system according to the present disclosure is a communication system, including a plurality of communication devices each of which is the above communication device on a network. 
     Advantageous Effect of Invention 
     The communication device or the like of the present disclosure makes it possible to prevent the absence of communication devices having started the DHCP function on the network if the communication devices having the DHCP server function are provided on the same network. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing the configuration of a system including communication devices according to embodiment 1. 
         FIG. 2  is a flowchart showing a specific example of the steps of confirming the presence or absence of a DHCP server on a network by the communication device of embodiment 1. 
         FIG. 3  is a flowchart showing a specific example of steps in which if the communication device of embodiment 1 stops a DHCP server function, the communication device determines whether to transmit a confirmation signal for confirming the presence or absence of the DHCP server on the network. 
         FIG. 4  is a sequence showing a specific example of the steps of determining the communication device for starting the DHCP server function to be performed by the communication devices provided in a communication system according to embodiment 1. 
         FIG. 5  is a block diagram showing the configuration of a system including communication devices according to embodiment 2. 
         FIG. 6  is a flowchart showing a specific example of the steps of calculating the transmission time of the confirmation signal for confirming the presence or absence of the DHCP server on the network by the communication device of embodiment 2. 
         FIG. 7  is a flowchart showing another specific example of the steps of calculating the transmission time of the confirmation signal for confirming the presence or absence of the DHCP server on the network by the communication device of embodiment 2. 
         FIG. 8  is a sequence showing another specific example of the steps of transmitting the confirmation signal by the communication device having stopped the DHCP server function to be performed by the communication devices provided in a communication system according to embodiment 2. 
         FIG. 9  is a block diagram showing the configuration of a system including communication devices according to embodiment 3. 
         FIG. 10  is a flowchart showing a specific example of the steps of switching the states of communications with an access point by the communication device of embodiment 3. 
         FIG. 11  is a block diagram showing the configuration of a system including communication devices according to another embodiment. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     (Fundamental Findings of the Present Disclosure) 
     The points of view of the inventors will be first described below. 
     DHCP servers are used to simplify the setting of IP addresses for client devices connected to a network. In some cases, a plurality of DHCP servers are provided on the same network. 
     On the same network, a communication device having a DHCP server function may act as a DHCP server instead of a fixed DHCP server acting as a permanent DHCP server. In this case, for example, the communication device broadcasts a confirmation signal to confirm the presence or absence of a DHCP server on the network. If it is determined that the DHCP server is absent on the network, the communication device acts as the DHCP server and controls the allocation of an IP address to each device on the network. If it is determined that a DHCP server is present on the network, the communication device does not act as a DHCP server and does not control the allocation of an IP address to each device on the network. 
     For example, in the presence of a plurality of communication devices capable of acting as DHCP servers on the same network, all the communication devices act as DHCP servers if the DHCP server is absent on the network at the transmission of the confirmation signal by the communication devices at the same time. 
     The communication devices capable of acting as DHCP servers repeatedly transmit the confirmation signal even after acting as DHCP servers, thereby determining the presence or absence of the DHCP server on the network. If it is determined that the DHCP server is present on the network, the communication devices do not act as DHCP servers and do not control the allocation of an IP address to each device on the network. This can prevent duplicate allocation of IP addresses to client devices in the presence of a plurality of DHCP servers on the same network. 
     In the presence of two communication devices acting as DHCP servers on the same network, if a DHCP server is present on the network, none of the communication devices acts as a DHCP server when the confirmation signal is transmitted at the same time. In other words, a DHCP server having the function of allocating an IP address to a client device is not present on the network. Thus, if a client device is added on the network, the device cannot obtain an IP address. 
     In view of the problem, the inventors have devised the present disclosure. 
     Embodiments will be specifically described below by optionally referring to the accompanying drawings. A detailed explanation may be omitted if necessary. For example, a detailed explanation of known matters or a redundant explanation of substantially the same configurations may be omitted. This will avoid an unnecessary explanation and enhance understanding of a person skilled in the art. 
     The inventors provide the accompanying drawings and the following explanation in order to sufficiently deepen the understanding of a person skilled in the art about the present disclosure. The inventors do not intend to limit the subject matters of claims. 
     Embodiment 1 
     Referring to  FIGS. 1 to 4 , embodiment 1 will be described below. 
     [1-1. Configuration] 
       FIG. 1  is a block diagram showing communication system  700  including communication device  100  according to embodiment 1. 
     As shown in  FIG. 1 , communication device (first communication device)  100  is installed in, for example, a house and is electrically connected to solar panel  500 , distribution switchboard  200  electrically connected to system power supply  600  acting as an external commercial power supply, and load device  300  such as an electrical appliance via power lines. Communication device  100  is connected to distribution switchboards  200  and  201  and load devices  300  and  301  via communication lines so as to communicate therewith. Communication device  100  is a communication device having a DHCP (Dynamic Host Configuration Protocol) server function that can allocate IP (Internet protocol) addresses to, for example, distribution switchboards  200  and  201  and load devices  300  and  301  on network  400 . Communication device  100  is a so-called house controller applied to a HEMS (Home Energy Management System) that acquires, for example, the power consumption of load device  300  or the like and notifies a user of communication device  100  of the power consumption. 
     Distribution switchboard  200  and load device  300  are client devices that can communicate with the communication device (first communication device  100  or second communication device  110 ) and acquire IP addresses from the communication device acting as a DHCP server. 
     Communications between communication device  100  and distribution switchboards  200  and  201  and load devices  300  and  301  are not limited to communications via communication lines. For example, communication device  100  may be connected to distribution switchboard  200  and load device  300  by radio communications. 
     Communication device  100  includes communicator (first communicator)  101  and controller (first controller)  102 . 
     Communicator  101  is a communication interface for communications between communication device  100  and distribution switchboards  200  and  201  and load devices  300  and  301 . Communicator  101  also acts as a communication interface for transmitting the confirmation signal for determining the presence or absence of other communication devices having started the DHCP server function on network  400 . The DHCP server function is a function of allocating an IP address to a client device connected to network  400 , for example, load device  300 . 
     Controller  102  is a controller for controlling the DHCP server function. 
     Controller  102  is implemented by, for example, a CPU (Central Processing Unit) and control programs stored in memory (not shown) such as ROM (Read Only Memory), RAM (Random Access Memory), an HDD (Hard Disk Drive), and an SSD (Solid State Drive) in communication device  100 . 
     For example, in response to an IP address request from load device  300 , controller  102  allocates IP addresses stored in the memory to load device  300 . Specifically, controller  102  transmits the IP address to load device  300  through communicator  101 . 
     Controller  102  transmits the confirmation signal to the other communication device (second communication device  110  in the present embodiment) through communicator  101 . Specifically, controller  102  broadcasts a first confirmation signal, which is an example of the confirmation signal, to network  400  through communicator  101 . If a first response signal is not received from second communication device  110  in response to the first confirmation signal, controller  102  determines that the other communication device having started the DHCP server function is absent on network  400 , and then starts the DHCP server function. 
     Furthermore, controller  102  transmits a second confirmation signal, which is another example of the confirmation signal, to the other communication device through communicator  101 . If a second response signal is received in response to the second confirmation signal, controller  102  stops the DHCP server function. Moreover, controller  102  transmits a third confirmation signal, which is another example of the confirmation signal, through communicator  101  to the other communication device that is the source of the received second response signal. 
     For example, if second communication device  110  having started the DHCP server function receives the second confirmation signal from first communication device  100 , the IP address allocated to first communication device  100  is transmitted as the second response signal. First communication device  100  uses the received IP address as the IP address of first communication device  100 . 
     Moreover, controller  102  controls the transmission of the third confirmation signal to the other communication device through communicator  101 , based on the second response signal. The second response signal includes the MAC (Media Access Control) address of the other communication device having transmitted the second response signal. When controller  102  having stopped the DHCP server function receives the second response signal, controller  102  further controls the transmission of the third confirmation signal through communicator  101 , according to the MAC address included in the second response signal. 
     For example, when the received MAC address is larger than the MAC address of communication device  100 , controller  102  further transmits the third confirmation signal through communicator  101 . Alternatively, when the received MAC address is smaller than the MAC address of communication device  100 , controller  102  further transmits the third confirmation signal through communicator  101 . 
     Thus, in the transmission control, the transmission of the third confirmation signal through communicator  101  is controlled under predetermined conditions according to the MAC address of communication device  100  and the MAC address of the source of the second response signal. 
     For example, when the received MAC address is larger than the MAC address of communication device  100 , controller  102  does not immediately transmit the third confirmation signal but transmits the third confirmation signal through communicator  101  after a predetermined time elapses. Alternatively, when the received MAC address is smaller than the MAC address of communication device  100 , controller  102  does not immediately transmit the third confirmation signal but transmits the third confirmation signal through communicator  101  after a predetermined time elapses. In this case, the predetermined time may be optionally determined in advance. For example, the predetermined time may be 20 seconds, 20 seconds or less, or 20 seconds or more. Alternatively, controller  102  may determine the predetermined time by calculating from the MAC address of controller  102  or generate a random number to determine the predetermined time from the generated random number as in, for example, communication device  100   a  according to embodiment 2, which will be discussed later. 
     If communication device  100  is the house controller, controller  102  controls devices on network  400 . For example, controller  102  obtains the power consumption of load device  300  or the like through communicator  101 . In this case, communication device  100  may further include a display (not shown) for notifying a user of communication device  100  of the power consumption. 
     Communication system  700  includes the communication devices of embodiment 1 on the same network  400 . The two communication devices are provided in communication system  700  according to the present embodiment. The number of communication devices is not particularly limited. In the following explanation, the two communication devices provided in communication system  700  according to the present embodiment will be referred to as first communication device  100  and second communication device  110 . 
     Second communication device  110  is disposed on network  400  like first communication device  100  and is connected to first communication device  100  while enabling communications therebetween. Second communication device  110  includes second communicator  111  and second controller  112 . Second communicator  111  is a communication interface having the same functional configuration as communicator (first communicator)  101 . Moreover, second controller  112  is a controller implemented by a CPU and a control program stored in memory (not shown) provided in second communication device  110 . Second controller  112  has the same functional configuration as controller (first controller)  102 . 
     Second communication device  110  is installed in, for example, the same house as first communication device  100  and is electrically connected to distribution switchboard  201 , which is electrically connected to system power supply  600 , and load device  301  such as an electrical appliance via power lines. Moreover, second communication device  110  is connected so as to communicate with distribution switchboards  200  and  201  and load devices  300  and  301  via communication lines. Second communication device  110  is a communication device having the DHCP server function that can allocate IP addresses to, for example, distribution switchboards  200  and  201  and load devices  300  and  301  on network  400 . 
     As described above, communication system  700  includes first communication device  100  and second communication device  110  that have the DHCP server function on the same network  400 . Communication system  700  is applied to, for example, a house controller system including a plurality of house controller as communication devices for controlling different electrical appliances in the homes of so-called two-family housing on the same network  400 . In this case, each of first communication device  100  and second communication device  110  can allocate IP addresses to distribution switchboard  200 , distribution switchboard  201 , load device  300 , and load device  301 . 
     [1-2. Operation] 
     The operations of communication system  700  including first communication device  100  and second communication device  110  that are configured thus will be described below. 
     First communication device  100  repeatedly transmits the first confirmation signal a predetermined number of times through first communicator  101 . If it is determined that the DHCP server is absent on network  400 , the DHCP server function is started such that first communication device  100  acts as a DHCP server. Furthermore, first communication device  100  repeatedly transmits the second confirmation signal the predetermined number of times through communicator  101 . If it is determined that the DHCP server is present on the network, the IP address allocated to communication device  100  from the DHCP server is acquired and the DHCP server function is stopped such that communication device  100  does not act as a DHCP server. Thereafter, first communication device  100  repeatedly transmits the third confirmation signal the predetermined number of times through communicator  101 . The number of times of transmission of the confirmation signal is not particularly limited and thus may be, for example, one, three, or five. Alternatively, the first confirmation signal, the second confirmation signal, and the third confirmation signal may be each repeatedly transmitted a different number of times. If the confirmation signal is transmitted the predetermined number of times and the response signal is not received in response to the confirmation signal, controller  102  determines that a fixed DHCP server or the other communication device having started the DHCP server function is absent on network  400 . The operations will be specifically discussed below. 
       FIG. 2  is a flowchart showing a specific example of the steps of confirming the presence or absence of the DHCP server on the network by the communication device of embodiment 1. First communication device  100  and second communication device  110  perform the same operations and thus only first communication device  100  will be discussed below. 
     Controller  102  broadcasts the first confirmation signal to network  400  through communicator  101  (step S 101 ). This allows controller  102  to confirm the presence or absence of the fixed DHCP server or the other communication device having started the DHCP server function on network  400 . Controller  102  repeatedly transmits, for example, DHCP DISCOVER as the first confirmation signal. 
     Subsequently, controller  102  determines whether or not the first response signal is received in response to the first confirmation signal (step S 102 ). If the first response signal is received (Yes at step S 102 ), controller  102  determines that the fixed DHCP server or the other communication device having started the DHCP server function is present on network  400 , and then controller  102  acquires the IP address included in the first response signal. 
     If the first response signal is not received (No at step S 102 ), controller  102  determines that the fixed DHCP server or the other communication device having started the DHCP server function is absent on network  400 , and then controller  102  starts the DHCP server function (step S 103 ). In other words, communication device  100  acts as the DHCP server. Moreover, in step S 103 , communication device  100  allocates one of the IP addresses of communication device  100  to communication device  100 . 
     Then, controller  102  broadcasts the second confirmation signal to network  400  through communicator  101  (step S 104 ). Controller  102  repeatedly transmits, for example, DHCP DISCOVER as the second confirmation signal. 
     Subsequently, controller  102  determines whether or not the second response signal is received in response to the second confirmation signal (step S 105 ). If the second response signal is not received (No at step S 105 ), controller  102  broadcasts the second confirmation signal to network  400  through communicator  101  (step S 104 ). 
     If the second response signal is received (Yes at step S 105 ), controller  102  determines that the other communication device having started the DHCP server function is present on network  400 , and then controller  102  acquires the IP address included in the second response signal and stops the DHCP server function (step S 106 ). For example, controller  102  transmits DHCP DISCOVER as the second confirmation signal through communicator  101  and receives, as the second response signal, DHCP OFFER including the MAC address of the source of the response signal and the IP address allocated to communication device  100 . If DHCP OFFER is received as the second response signal, controller  102  transmits DHCP REQUEST for requesting permission to use the received IP address, to the MAC address of the source through communicator  101 . If DHCP ACK is received as the response signal of DHCP REQUEST, controller  102  acquires and uses the received IP address as the IP address of communication device  100 . In other words, in step S 103 , communication device  100  does not use the IP address allocated to communication device  100  but uses the IP address received in step S 105 . 
     Subsequently, controller  102  controls the transmission of the third confirmation signal to the MAC address of the source of the second response signal through communicator  101  based on the second response signal received in step S 105  (step S 107 ). Specifically, in step S 107 , controller  102  determines whether to transmit, for example, DHCP REQUEST as the third confirmation signal based on the MAC address included in the second response signal. Thus, controller  102  determines whether or not the other communication device having started the DHCP server function on network  400  currently acts as the DHCP server. 
     The operations are also performed by second communication device  110 . 
     In the following explanation, if the DHCP server function is stopped, controller  102  determines whether to transmit the third confirmation signal for confirming the presence or absence of the DHCP server on network  400 . 
       FIG. 3  is a flowchart showing a specific example of steps in which if the communication device of embodiment 1 stops the DHCP server function, the communication device determines whether to transmit the third confirmation signal for confirming the presence or absence of the DHCP server on the network. 
     Controller  102  performs the operations of steps S 101  to S 105  in  FIG. 2 . 
     If the second response signal is received (Yes at step S 105  in  FIG. 2 ), controller  102  determines that the other communication device having started the DHCP server function is present on network  400 , and then controller  102  acquires the IP address included in the second response signal and stops the DHCP server function (step S 106 ). 
     Controller  102  then compares the MAC address of controller  102  (communication device  100 ) and the MAC address of the source of the second response signal. Specifically, controller  102  determines whether or not the MAC address of controller  102  is larger than the MAC address of the source of the second response signal (step S 201 ). 
     If controller  102  determines that the MAC address of controller  102  is larger than the MAC address of the source of the second response signal (Yes at step S 201 ), controller  102  immediately transmits, through communicator  101 , the third confirmation signal to the MAC address of the source of the second response signal received in step S 105  (step S 202 ). 
     If controller  102  determines that the MAC address of controller  102  is smaller than the MAC address of the source of the second response signal (No at step S 201 ), controller  102  transmits the third confirmation signal after the elapse of a predetermined time (step S 202   a ). 
     The operations are also performed by second communication device  110 . The MAC address is stored in advance as a different numeric value for each device. This continuously transmits the confirmation signal and the response  20 , signal between the communication devices provided in communication system  700 , thereby preventing the repetition of the start and stop of the DHCP server function. 
     In step S 201 , controller  102  determines whether or not the MAC address of controller  102  is larger than the MAC address of the source of the second response signal. The present invention is not limited to this process. In step S 201 , controller  102  may determine whether or not the MAC address of controller  102  is smaller than the MAC address of the source of the second response signal. In this case, if controller  102  in step S 202  determines that the MAC address of controller  102  is smaller than the MAC address of the source of the second response signal, controller  102  immediately transmits, through communicator  101 , the third confirmation signal to the MAC address of the source of the second response signal received in step S 105 . If controller  102  in step S 202   a  determines that the MAC address of controller  102  is larger than the MAC address of the source of the second response signal, controller  102  transmits the third confirmation signal after a predetermined time. The criterion may be determined in advance for each of the communication devices provided in communication system  700 . 
       FIG. 4  is a sequence showing a specific example of the steps of determining the communication device for starting the DHCP server functions performed by the communication devices provided in communication system  700  according to embodiment 1. In  FIG. 4 , in substantially the same steps as those of the flowcharts of  FIGS. 2 and 3 , the same reference numerals are used and the explanation of the steps may be omitted. 
     First communication device  100  and second communication device  110  repeatedly broadcast the first confirmation signal to network  400  (step S 101 ). Thus, first communication device  100  and second communication device  110  confirm the presence or absence of fixed DHCP server  800  having the DHCP server function started all the time or the other communication device having started the DHCP server function on network  400 . In embodiment 1, fixed DHCP server  800  is absent on network  400 . 
     Subsequently, first communication device  100  and second communication device  110  determine whether or not the first response signal is received (step S 102  in  FIG. 2 ). 
     Thereafter, if the first response signal is not received (No at step S 102  in  FIG. 2 ), first communication device  100  and second communication device  110  determine that fixed DHCP server  800  or the other communication device having started the DHCP server function is absent on network  400 , and then the communication devices start the DHCP server function (step S 103 ). 
     First communication device  100  and second communication device  110  then broadcast the second confirmation signal to network  400  (step S 104 ). 
     If the second response signal is received (step S 105   a ), first communication device  100  and second communication device  110  determine that the other communication device having started the DHCP server function is present on network  400 , and then the communication devices acquire the IP address included in the second response signal and stops the DHCP server function (step S 106 ). 
     First communication device  100  and second communication device  110  then compare the MAC addresses of the communication devices and the MAC address of the source of the second response signal. For example, first communication device  100  determines whether or not the MAC address of first communication device  100  is larger than the MAC address of second communication device  110  (step S 201 ). Likewise, in step S 201 , second communication device  110  determines whether or not the MAC address of second communication device  110  is larger than the MAC address of first communication device  100 . In the present embodiment, a criterion is stored in first communication device  100  and second communication device  110  so as to transmit the third confirmation signal from one of the communication devices with a larger MAC address. In the following explanation, the MAC address of first communication device  100  is smaller than the MAC address of second communication device  110 . 
     In step S 201 , first communication device  100  determines that the MAC address of first communication device  100  is smaller than the MAC address of second communication device  110  and completes the operation. 
     Moreover, in step S 201 , second communication device  110  determines that the MAC address of second communication device  110  is larger than the MAC address of first communication device  100  and repeatedly transmits the third confirmation signal to first communication device  100  (step S 202 ). 
     Thereafter, if the third response signal is not received in response to the third confirmation signal, second communication device  110  determines that first communication device  100  has not started the DHCP server function and starts the DHCP server function (step S 203 ). 
     Thus, if distribution switchboard  200  broadcasts, for example, an IP address request to network  400  (step S 204 ), second communication device  110  can allocate an IP address to distribution switchboard  200  (step S 205 ). In this way, distribution switchboard  200  can acquire the IP address (step S 206 ). 
     If communication device  100  determines that the MAC address of communication device  100  is smaller than the MAC address of the source of the second response signal, communication device  100  transmits the third confirmation signal after the predetermined time (step S 202   a ). Thus, from second communication device  110  having started the DHCP server function, first communication device  100  receives the third response signal in response to the third confirmation signal (step S 202   b ), so that first communication device  100  determines that second communication device  110  has started the DHCP server, and does not start the DHCP server function of first communication device  100 . 
     [1-3. Effects] 
     As described above, in this embodiment, communication device  100  is a communication device in communication system  700  including a plurality of communication devices on network  400 , the plurality of communication devices each having a Dynamic Host Configuration Protocol (DHCP) server function of allocating an Internet Protocol (IP) address. Communication device  100  includes communicator  101  that transmits a confirmation signal for confirming presence or absence of an other communication device having the DHCP server function started on network  400 . Communication device  100  also includes controller  102  that controls the DHCP server function. Controller  102 : transmits a first confirmation signal to the other communication device through communicator  101 ; and starts the DHCP server function and transmits a second confirmation signal to the other communication device through communicator  101  when a first response signal is not received from the other communication device in response to the first confirmation signal. In contrast, controller  102 ; stops the DHCP server function when a second response signal is received in response to the second confirmation signal; and performs a transmission control of a third confirmation signal to the other communication device through communicator  101 , based on the second response signal. 
     With this configuration, controller  102  can continuously confirm whether or not the other communication device having started the DHCP server function on network  400  continuously acts as a DHCP server. This can prevent the absence of communication devices having started the DHCP server function on network  400 . 
     Furthermore, the second response signal includes a Media Access Control (MAC) address of the other communication device which has transmitted the second response signal, and controller  102  may perform the transmission control according to the MAC address. 
     Thus, even if a plurality of communication devices having identical configurations are present on the same network  400 , the devices have different MAC addresses as criteria for controlling the transmission of the third confirmation signal, enabling simple transmission control in different ways. 
     For example, during the transmission control of the third confirmation signal, controller  102  may transmit the third confirmation signal through communicator  101  when the MAC address of second communication device  110  is larger than the MAC address of first communication device  100 . Furthermore, for example, during the transmission control of the third confirmation signal, controller  102  may transmit the third confirmation signal through communicator  101  after an elapse of a predetermined time when the MAC address of second communication device  110  is smaller than the MAC address of communication device  100 . 
     Furthermore, for example, during the transmission control of the third confirmation signal, controller  102  may transmit the third confirmation signal through communicator  101  when the MAC address of second communication device  110  is smaller than the MAC address of communication device  100 . Furthermore, for example, during the transmission control of the third confirmation signal, controller  102  may transmit the third confirmation signal through communicator  101  after an elapse of a predetermined time when the MAC address of second communication device  110  is larger than the MAC address of communication device  100 . 
     With this configuration, even if a plurality of communication devices (first communication device  100  and second communication device  110  in embodiment 1) on the same network  400 , any one of the communication devices starts the DHCP server function. Furthermore, even if the DHCP server functions of two or more of the communication devices are stopped, any one of the communication devices confirms the communication device having started the DHCP server function on network  400 . Thus, all of the communication devices on the same network  400  do not need to transmit the third confirmation signal for confirming the communication device having started the DHCP server function. This can prevent the traffic of the communication devices provided in communication system  700 . 
     With this configuration, communication device  100  can switch between the transmission of the third confirmation signal immediately in response to the MAC address and the transmission of the third confirmation signal after the elapse of the predetermined time instead of immediately transmitting the third confirmation signal. Thus, with this configuration, even if the communication device to immediately transmit the third confirmation signal does not transmit the third confirmation signal due to, for example, an error in communication system  700  including the communication devices according to embodiment 1, the other communication devices transmit the third confirmation signal after the elapse of the predetermined time. Thus, communication system  700  can prevent the simultaneous starting of the DHCP server functions by the communication devices according to embodiment 1 and the absence of the communication device having started the DHCP server function. 
     Moreover, in the present embodiment, communication system  700  includes multiple communication devices  100  on the same network  400 . Communication system  700  includes first communication device  100  and second communication device  110  in the present embodiment. 
     In other words, in communication system  700 , the control function provided in the case where multiple communication devices having the DHCP server functions exist on the same network  400  can prevent the absence of the communication device having started the DHCP server function on network  400 . 
     Embodiment 2 
     Referring to  FIGS. 5 to 8 , embodiment 2 will be described below. Substantially the same configurations as embodiment 1 are indicated by the same reference numerals and the redundant explanation thereof may be omitted or simplified. 
     [2-1. Configuration] 
       FIG. 5  is a block diagram showing the configuration of a system including communication devices according to embodiment 2. 
     Communication device (first communication device)  100   a  is electrically connected to distribution switchboard  200 , which is electrically connected to solar panel  500  and system power supply  600 , and load device  300  such as an electrical appliance via power lines. Communication device  100   a  is connected so as to communicate with distribution switchboards  200  and  201  and load devices  300  and  301  via communication lines. Communication device  100   a  has a DHCP server function and can allocate IP addresses to, for example, distribution switchboards  200  and  201  and load devices  300  and  301  on network  400 . Communication device  100   a  is a so-called house controller applied to a HEMS that acquires, for example, the power consumption of load device  300  or the like and notifies a user of communication device  100   a  of the power consumption. 
     Distribution switchboards  200  and  201  and load devices  300  and  301  are client devices that acquire IP addresses from one of first communication device  100   a  or second communication device  110   a , each having started the DHCP server function. 
     Communications between communication device  100   a  and distribution switchboards  200  and  201  and load devices  300  and  301  are not limited to communications via communication lines. For example, communication device  100   a  may be connected to distribution switchboards  200  and  201  and load devices  300  and  301  by radio communications. 
     Communication device  100   a  includes communicator (first communicator)  101   a  and controller (first controller)  102   a.    
     Like communicator  101 , communicator  101   a  acts as a communication interface for communications between communication device  100   a  and the other communication device (e.g., second communication device  110   a ), distribution switchboard  200 , and load device  300  or the like on network  400 . 
     Controller  102   a  is a controller for controlling the DHCP server function. Controller  102   a  is implemented by, for example, a CPU and control programs stored in memory (not shown) such as ROM, RAM, an HDD, and an SSD in communication device  100   a.    
     For example, in response to an IP address request from load device  300 , controller  102   a  allocates IP addresses stored in the memory to load device  300 . 
     Controller  102   a  transmits a first confirmation signal to the other communication device (second communication device  110   a  in embodiment 2) through communicator  101   a . Specifically, controller  102   a  broadcasts a first confirmation signal to network  400  through communicator  101   a . If a first response signal is not received from second communication device  110   a , controller  102   a  determines that the other communication device having started the DHCP server function is absent on network  400 , and then starts the DHCP server function. 
     Furthermore, controller  102   a  transmits a second confirmation signal to the other communication device through communicator  101   a . If a second response signal is received, controller  102   a  stops the DHCP server function. Specifically, controller  102   a  broadcasts the second confirmation signal to network  400  through communicator  101   a . If the second response signal is received, controller  102   a  stops the DHCP server function. 
     Moreover, controller  102   a  controls the transmission of a third confirmation signal to the other communication device through communicator  101 , based on the second response signal. Specifically, controller  102   a  transmits the third confirmation signal to the other communication device through communicator  101   a  after the elapse of a predetermined time from the reception time of the second response signal. 
     For example, if the DHCP server function is stopped, controller  102   a  transmits the third confirmation signal through communicator  101   a  after the elapse of a time corresponding to the MAC address of communication device  100   a  from the reception of the second response signal. The predetermined time is determined in advance for each of the communication devices of embodiment 2 according to the MAC address. 
     For example, if the DHCP server function is stopped, controller  102   a  receives the second response signal and then generates a random number to determine the predetermined time according to the random number. In short, controller  102   a  may generate a random number and determine the predetermined time according to the random number. For example, controller  102   a  may generate a five-digit random number from the MAC address of communication device  100   a  and determine the milliseconds of the generated five-digit random number as the predetermined time. 
     Controller  102   a  may include a timer (not shown), e.g., a RTC (Real Time Clock) for measuring the predetermined time. 
     Communication system  700   a  includes the communication devices of embodiment 2 on the same network  400 . The number of communication devices is two in communication system  700   a  according to embodiment 2. The number of communication devices is not particularly limited. In the following explanation, the two communication devices provided in communication system  700   a  according to the present embodiment will be referred to as first communication device  100   a  and second communication device  110   a.    
     Second communication device  110   a  is disposed on network  400  like first communication device  100   a  and is connected so as to communicate with first communication device  100   a . Second communication device  110   a  includes second communicator  111   a  and second controller  112   a . Second communicator  111   a  is a communication interface having the same functional configuration as communicator (first communicator)  101   a . Moreover, second controller  112   a  is a controller implemented by a CPU and a control program stored in memory (not shown) provided in second communication device  110   a . Second controller  112   a  has the same functional configuration as controller (first controller)  102   a.    
     Second communication device  110   a  is installed in, for example, the same house as first communication device  100   a  and is electrically connected to distribution switchboard  201 , which is electrically connected to system power supply  600 , and load device  301  such as an electrical appliance via power lines. Moreover, second communication device  110   a  is connected so as to communicate with distribution switchboards  200  and  201  and load devices  300  and  301  via communication lines. Second communication device  110   a  has the DHCP server function and can allocate IP addresses to, for example, distribution switchboard  201  and load device  301  on network  400 . 
     [2-2. Operation] 
     First communication device  100   a  and second communication device  110   a  having the above configurations are provided in communication system  700   a . The operations of communication system  700   a  will be discussed below. The same steps as those of embodiment 1 are indicated by the same numerals and the explanation thereof may be omitted. 
     First communication device  100   a  transmits the first confirmation signal through first communicator  101   a . If it is determined that the DHCP server is absent on network  400 , the DHCP server function is started such that first communication device  100   a  acts as a DHCP server. Furthermore, first communication device  100   a  repeatedly transmits the second confirmation signal through communicator  101 . If it is determined that the DHCP server is present on network  400 , the IP address allocated to communication device  100   a  from the DHCP server is acquired and the DHCP server function is stopped to prevent communication device  100   a  from acting as a DHCP server. Moreover, the third confirmation signal is transmitted through communicator  101   a  after the elapse of the predetermined time. The operations will be specifically discussed below. 
       FIG. 6  is a flowchart showing a specific example of the steps of calculating the transmission time of the confirmation signal for confirming the presence or absence of the DHCP server on network  400  by the communication device of embodiment 2. First communication device  100   a  and second communication device  110   a  perform the same operations and thus only first communication device  100   a  will be discussed below. 
     Controller  102   a  broadcasts the first confirmation signal to network  400  through communicator  101   a  (step S 101 ). This allows controller  102   a  to confirm the presence or absence of the fixed DHCP server or the other communication device having started the DHCP server function on network  400 . Controller  102   a  repeatedly transmits, for example, DHCP DISCOVER as the first confirmation signal. 
     Subsequently, controller  102   a  performs the operations of steps S 102  to S 105  in  FIG. 2 . 
     If the second response signal is received (Yes at step S 105  in  FIG. 2 ), controller  102   a  determines that the other communication device having started the DHCP server function is present on network  400 , and then controller  102  acquires the IP address included in the second response signal and stops the DHCP server function (step S 106 ). 
     Controller  102   a  then determines a time until the transmission of the third confirmation signal, from the MAC address of controller  102   a  (communication device  100   a ) (step S 301 ). 
     Subsequently, controller  102   a  transmits the third confirmation signal through communicator  101   a  after the elapse of the predetermined time that is determined in step S 301  (step S 302 ). 
     The operations are also performed by second communication device  110   a . Thus, the communication devices provided in communication system  700   a  can transmit the third confirmation signal at different times if the DHCP server functions are simultaneously stopped. 
       FIG. 7  is a flowchart showing another specific example of the steps of calculating the transmission time of the confirmation signal for confirming the presence or absence of the DHCP server on network  400  by the communication device of embodiment 2. First communication device  100   a  and second communication device  110   a  perform the same operations and thus only first communication device  100   a  will be discussed below. 
     Controller  102   a  broadcasts the first confirmation signal to network  400  through communicator  101   a  (step S 101 ). 
     Subsequently, controller  102   a  performs the operations of steps S 102  to S 105  in  FIG. 2 . 
     If the second response signal is received (Yes at step S 105  in  FIG. 2 ), controller  102   a  determines that the other communication device having started the DHCP server function is present on network  400 , and then controller  102  acquires the IP address included in the response signal and stops the DHCP server function (step S 106 ). 
     Thereafter, controller  102   a  generates a random number (step S 401 ). For example, controller  102   a  in step S 401  generates a random number according to the MAC address of communication device  100 . 
     Controller  102   a  then determines a time until the transmission of the third confirmation signal, from the random number generated in step S 401  (step S 402 ). Specifically, controller  102   a  in step S 402  determines a time from the reception of the second response signal from the other communication device to the transmission of the third confirmation signal, based on the random number generated in step S 401 . 
     Subsequently, controller  102   a  transmits the third confirmation signal through communicator  101   a  after the elapse of the time determined in step S 402  (step S 302 ). 
     The operations are also performed by second communication device  110   a . Also in this case, the communication devices provided in communication system  700   a  can transmit the confirmation signal at different times if the DHCP server functions are simultaneously stopped. 
       FIG. 8  is a sequence showing another specific example of the steps of transmitting the confirmation signal by the communication device having stopped the DHCP server function to be performed by the communication devices provided in communication system  700   a  according to embodiment 2. In  FIG. 8 , in substantially the same steps as those of the flowcharts of  FIGS. 2, 6, and 7 , the same reference numerals are used and the explanation of the steps may be omitted. 
     First communication device  100   a  and second communication device  110   a  repeatedly broadcast the first confirmation signal to network  400  (step S 101 ). Thus, first communication device  100   a  and second communication device  110   a  confirm the presence or absence of fixed DHCP server  800  or the other communication device having started the DHCP server function on network  400 . In embodiment 2, fixed DHCP server  800  is absent on network  400 . 
     Subsequently, first communication device  100   a  and second communication device  110   a  determine whether or not the first response signal is received (step S 102  in  FIG. 2 ). 
     Thereafter, if the first response signal is not received (No at step S 102  in  FIG. 2 ), first communication device  100   a  and second communication device  110   a  determine that fixed DHCP server  800  or the other communication device having started the DHCP server function is absent on network  400 , and then the communication devices start the DHCP server function (step S 103 ). 
     First communication device  100   a  and second communication device  110   a  then broadcast the second confirmation signal to network  400  (step S 104 ). 
     If the second response signal is received (step S 105   a ), first communication device  100   a  and second communication device  110   a  determine that the other communication device having started the DHCP server function is present on network  400 , and then the communication devices acquire the IP address included in the second response signal and stop the DHCP server function (step S 106 ). 
     Subsequently, first communication device  100   a  and second communication device  110   a  each determine a time until the transmission of the third confirmation signal (step S 501 ). Specifically, first communication device  100   a  and second communication device  110   a  in step S 501  each determine the time until the transmission of the third confirmation signal, by performing step S 301  in  FIG. 6 . Alternatively, first communication device  100   a  and second communication device  110   a  in step S 501  each determine the time until the transmission of the third confirmation signal, by performing step S 401  and step S 402  in  FIG. 7 . 
     Subsequently, first communication device  100   a  and second communication device  110   a  each determine a predetermined time until the transmission of the confirmation signal, from the random number generated in step S 401  (step S 402 ). 
     First communication device  100   a  and second communication device  110   a  then transmit the third confirmation signal after the elapse of the time determined in step S 501 . In  FIG. 7 , the time determined by second communication device  110   a  is shorter than that of first communication device  100   a . Second communication device  110   a  repeatedly transmits the third confirmation signal to first communication device  100   a  after the elapse of the time determined in step S 501  (step S 502 ). 
     Thereafter, if the third response signal is not received, second communication device  110   a  determines that first communication device  100   a  has not started the DHCP server function, and starts the DHCP server function (step S 503 ). 
     First communication device  100   a  transmits the third confirmation signal to second communication device  110   a  after the elapse of the time determined in step S 501  (step S 504 ). 
     At this point, second communication device  110   a  has started the DHCP server function in step S 503  and thus transmits the third response signal to first communication device  100   a  (step S 505 ). 
     First communication device  100   a  receives the third response signal from second communication device  110   a . Thus, first communication device  100   a  does not start the DHCP server function. 
     In step S 504 , the third confirmation signal transmitted by first communication device  100   a  and second communication device  110   a  may be DHCP DISCOVER for requesting an IP address but is preferably DHCP REQUEST for requesting permission to use the IP address acquired in step S 105   a . In step S 505 , second communication device  110   a  may transmit DHCP ACK to first communication device  100   a  as the third response signal in response to DHCP REQUEST. Thus, another IP address is not allocated to first communication device  100   a . This prevents the acquisition of duplicate IP addresses by first communication device  100   a.    
     [2-3. Effects] 
     As described above, in communication system  700   a  according to embodiment 2, first communication device  100   a  and second communication device  110   a  transmit the third confirmation signal at different times. 
     For example, during the transmission control of the third confirmation signal, controller  102   a  may transmit the third confirmation signal through communicator  101  after an elapse of a time corresponding to a MAC address of communication device  100   a  from the reception of the second response signal. 
     With this configuration, even if a plurality of communication devices (first communication device  100   a  and second communication device  110   a  in embodiment 2) on the same network  400 , any one of the communication devices starts the DHCP server function prior to the other. This prevents simultaneous starting of the DHCP server functions of the communication devices provided in communication system  700   a.    
     Furthermore, for example, during the transmission control of the third confirmation signal, controller  102   a  may generate a random number and transmit the third confirmation signal through communicator  101  after an elapse of a time corresponding to the random number. 
     With this configuration, even if a plurality of communication devices on the same network  400 , any one of the communication devices is likely to start the DHCP server function prior to the other. This further prevents simultaneous starting of the DHCP server functions of the communication devices provided in communication system  700   a.    
     Embodiment 3 
     Referring to  FIG. 9 , embodiment 3 will be described below. Substantially the same configurations as embodiments 1 and 2 are indicated by the same reference numerals and the redundant explanation thereof may be omitted or simplified. 
     [3-1. Configuration] 
       FIG. 9  is a block diagram showing the configuration of a system including communication devices according to embodiment 3. 
     Communication device (first communication device)  100   b  is electrically connected to distribution switchboard  200 , which is electrically connected to solar panel  500  and system power supply  600 , and load device  300  such as an electrical appliance via power lines. Communication device  100   b  is connected so as to communicate with distribution switchboards  200  and  201  and load devices  300  and  301  via communication lines. Communication device  100   b  has a DHCP server function and can allocate IP addresses to distribution switchboards  200  and  201  and load devices  300  and  301  on network  400 . Communication device  100   b  is a so-called house controller applied to a HEMS that acquires, for example, the power consumption of load device  300  or the like and notifies a user of communication device  100   b  of the power consumption. 
     Distribution switchboards  200  and  201  and load devices  300  and  301  are client devices that acquire IP addresses from one of first communication device  100   b  or second communication device  110   b  described later, each having started the DHCP server function. 
     Communications between communication device  100   b  and distribution switchboards  200  and  201  and load devices  300  and  301  are not limited to communications via communication lines. For example, communication device  100   b  may be connected to distribution switchboards  200  and  201  and load devices  300  and  301  by radio communications. 
     Communication device  100   b  is connected so as to communicate with access point  900 . Access point  900  can communicate with external network  410 , e.g., a public wireless LAN (Local Area Network) or the Internet unlike network  400 . In other words, communication device  100   b  is connected so as to communicate with the external network via access point  900 . Specifically, communicator (first communicator)  101   b  is connected so as to communicate with external network  410  different from network  400 . 
     Access point  900  is a communication device that enables communication device  100   b  to communicate with external network  410 . Communication device  100   b  and access point  900  have the communication function of, for example, Wi-Fi (registered trademark). Communication device  100   b  and access point  900  are connected so as to communicate with each other. 
     Communications of communication device  110   b  with external network  410  are enabled or disabled depending on whether the DHCP server function has started or not. 
     Communications between communication device  100   b  and access point  900  are not limited to wireless communications. For example, communication device  100   b  and access point  900  may be connected to each other through wire communications via communication lines. 
     Communication device  100   b  includes communicator (first communicator)  101   b  and controller (first controller)  102   b.    
     Like communicator  101 , communicator  101   b  acts as a communication interface for communications between communication device  100   b  and the other communication device (e.g., second communication device blob), distribution switchboard  200 , and load device  300  on network  400 . Moreover, communicator  101   b  is connected so as to communicate with external network  410  different from network  400 . 
     Controller  102   b  is a controller for controlling the DHCP server function. Controller  102   b  is implemented by, for example, a CPU and control programs stored in memory (not shown) such as ROM, RAM, an HDD, and an SSD in communication device  100   b.    
     For example, in response to an IP address request from load device  300 , controller  102   b  allocates IP addresses stored in the memory to load device  300 . 
     Controller  102   b  transmits a first confirmation signal to the other communication device (second communication device  110   b  in the present embodiment) through communicator  101   b . Specifically, controller  102   b  broadcasts the first confirmation signal to network  400  through communicator  101   b . If a first response signal is not received from second communication device  110   b , controller  102   b  determines that the other communication device having started the DHCP server function is absent on network  400 , and then starts the DHCP server function. 
     Furthermore, controller  102   b  transmits a second confirmation signal to the other communication device through communicator  101   a . If a second response signal is received, controller  102   b  stops the DHCP server function. Specifically, controller  102   b  broadcasts the second confirmation signal to network  400  through communicator  101   b . If the second response signal is received, controller  102   b  stops the DHCP server function. 
     Moreover, controller  102   b  controls the transmission of a third confirmation signal to the other communication device through communicator  101   b , based on the second response signal. For example, like controller  102  according to embodiment 1 or controller  102   a  according to embodiment 2, controller  102   b  transmits the third confirmation signal to the other communication device through communicator  101   b  if controller  102   b  determines that the third confirmation signal is to be transmitted based on the MAC address of controller  102   b  or after the elapse of a predetermined time from the reception time of the second response signal. 
     If the DHCP server function is started, controller  102   b  provided in communication device  100   b  according to embodiment 3 allows communicator  101   b  to communicate with external network  410 . If the DHCP server function is not started, controller  102   b  does not allow communicator  101   b  to communicate with external network  410 . 
     Communication system  700   b  includes the communication devices of embodiment 3 on the same network  400 . The number of communication devices is two in communication system  700   b  according to embodiment 3. The number of communication devices is not particularly limited. In the following explanation, the two communication devices provided in communication system  700   b  according to embodiment 3 will be referred to as first communication device  100   b  and second communication device  110   b.    
     Second communication device  110   b  is disposed on network  400  like first communication device  100   b  and is connected so as to communicate with first communication device  100   b . Second communication device  110   b  includes second communicator  111   b  and second controller  112   b . Second communicator  111   b  is a communication interface having the same functional configuration as communicator (first communicator)  101   b . Moreover, second controller  112   b  is a controller implemented by a CPU and a control program stored in memory (not shown) provided in second communication device  110   b . Second controller  112   b  has the same functional configuration as controller (first controller)  102   b.    
     Second communication device  110   b  is installed in, for example, the same house as first communication device  100   b  and is electrically connected to distribution switchboard  201 , which is electrically connected to system power supply  600 , and load device  301  such as an electrical appliance via power lines. Moreover, second communication device  110   b  is connected so as to communicate with distribution switchboards  200  and  201  and load devices  300  and  301  via communication lines. Second communication device  110   b  has the DHCP server function and can allocate IP addresses to, for example, distribution switchboard  201  and load device  301  on network  400 . 
     Like first communication device  100   b , second communication device  110   b  is connected so as to communicate with access point  900 . Access point  900  can communicate with external network  410 , e.g., the Internet that is different from network  400 . In other words, second communication device  110   b  is connected so as to communicate with external network  410  via access point  900 . Specifically, communicator  111   b  is connected so as to communicate with external network  410  different from network  400 . 
     Communications of communication device  110   b  with external network  410  are enabled or disabled depending on whether the DHCP server function has been started or not. Specifically, if the DHCP server function is started, controller  112   b  allows communicator  111   b  to communicate with external network  410  like controller  102   b . If the DHCP server function is not started, controller  112   b  does not allow communicator  111   b  to communicate with external network  410 . Second communication device  110   b  and access point  900  have the communication function of, for example, Wi-Fi (registered trademark). Second communication device  110   b  and access point  900  are connected so as to communicate with each other. 
     Communications between second communication device  110   b  and access point  900  are not limited to wireless communications. For example, second communication device  110   b  and access point  900  may be connected to each other by wire communications via communication lines. 
     [3-2. Operation] 
     First communication device  100   b  and second communication device  110   b  having the above configurations are provided in communication system  700   b . The operations of communication system  700   b  will be discussed below. The same steps as those of embodiment 1 are indicated by the same numerals and the explanation thereof may be omitted. 
       FIG. 10  is a flowchart showing a specific example of the steps of switching states of communications with access point  900  by the communication device of embodiment 3. First communication device  100   b  and second communication device  110   b  perform the same operations and thus only first communication device  100   b  will be discussed below. 
     Controller  102   b  broadcasts the first confirmation signal to network  400  through communicator  101   b  (step S 101 ). 
     Subsequently, controller  102   b  determines whether or not the first response signal is received in response to the first confirmation signal (step S 102 ). If the first response signal is received (Yes at step S 102 ), controller  102   b  determines that the other communication device having started the DHCP server function is present, and then controller  102   b  acquires the IP address included in the first response signal. 
     Since controller  102   b  determines that the other communication device has started the DHCP server, controller  102   b  does not start the DHCP server function and disables communications of communicator  101   b  with external network  410  (step S 601 ). Specifically, in step S 601 , controller  102   b  prohibits communicator  101   b  from communicating with access point  900 , disabling communications with external network  410 . In this case, communicator  101   b  is controlled to disable communications with external network  410 . This control prohibits, for example, signal transmission to access point  900  and the processing of the signal from access point  900 . The specific contents of the control are not particularly limited as long as communications with external network  410  are disabled or are not carried out. 
     If the first response signal is not received (No at step S 102 ), controller  102   b  determines that the fixed DHCP server or the other communication device having started the DHCP server function is absent on network  400 , and then controller  102   b  starts the DHCP server function (step S 103 ). 
     Since the DHCP server is started, controller  102   b  enables communications of communicator  101   b  with external network  410  (step S 602 ). Specifically, in step S 602 , controller  102   b  enables the transmission and reception of signals to and from access point  900  and the processing of the signal received from access point  900 . In this way, controller  102   b  enables or disables communications between communicator  101   b  and access point  900  depending on whether the DHCP server function has been started or not. 
     Then, controller  102   b  broadcasts the second confirmation signal to network  400  through communicator  101   b  (step S 104 ). 
     Subsequently, controller  102   b  determines whether or not the second response signal is received in response to the second confirmation signal (step S 105 ). If the second response signal is not received (No at step S 105 ), controller  102   b  broadcasts the second confirmation signal to network  400  through communicator  101  (step S 104 ). 
     If the second response signal is received (Yes at step S 105 ), controller  102   b  determines that the other communication device having started the DHCP server function is present on network  400 , and then controller  102   b  acquires the IP address included in the second response signal and stops the DHCP server function (step S 106 ). 
     Controller  102   b  then stops the DHCP server function in step S 106  so as to disable communications of communicator  101   b  with external network  410  (step S 603 ). 
     Subsequently, controller  102   b  controls the transmission of the third confirmation signal to the MAC address of the source of the second response signal through communicator  101   b  based on the second response signal received in step S 105  (step S 107 ). In step S 107 , for example, controller  102   b  may perform the operations of steps S 201  and S 202  or S 202   a  in  FIG. 3  like controller  102  according to embodiment 1. In step S 107 , for example, controller  102   b  may perform the operations of steps S 301  and S 302  in  FIG. 6  or the operations of steps S 401  to S 302  in  FIG. 7  like controller  102   a  according to embodiment 2. 
     The operations are also performed by second communication device  110   b.    
     [3-3. Effects] 
     In communication system  700   b  according to embodiment 3, communication device  100   b  according to embodiment 3 is added to the functional configuration of communication device  100  according to embodiment 1. Furthermore, communicator  101   b  in communication device  100   b  is connected to external network  410  different from network  400 . Controller  102   b  in communication device  100   b  enables communications of communicator  101   b  with external network  410  when the DHCP server function is started, and disables communications of communicator  101   b  with external network  410  when the DHCP server function is not started. 
     With this configuration, if a plurality of communication devices (first communication device  100   b  and second communication device  110   b  in embodiment 3) are connected to access point  900  connected to external network  410  on the same network  400 , only any one of the communication devices enables communications with external network  410 . Thus, this configuration can prevent simultaneous communications of the communication devices in communication system  700   b  with external network  410 , thereby preventing crosstalk at, for example, access point  900 . This configuration further prevents unnecessary signal transmission and reception, for example, the transmission of a signal to access point  900  after the same signal is received by the communication devices from the same load device (e.g., load device  300 ) or the reception of a signal by the communication devices after the signal is received by access point  900  through external network  410 . 
     Other Embodiments 
     Embodiments 1, 2, and 3 were illustrated as techniques disclosed in the present application. However, the techniques of the present disclosure are not limited to the embodiments. The present disclosure is also applicable to the embodiments optionally involving modification, replacement, addition, and omission. The constituent elements described in the embodiments may be combined to achieve another embodiment. 
     Other embodiments will be discussed below. 
     For example, in embodiment 1, first communication device  100  and second communication device  110  in communication system  700  are electrically connected to different distribution switchboards (distribution switchboard  200  and distribution switchboard  201 ). However, the communication system according to the present embodiment is not limited to this configuration. 
       FIG. 11  is a block diagram showing the configuration of a system including communication devices according to another embodiment. 
     As shown in  FIG. 11 , unlike in embodiments 1, 2, and 3, first communication device  100  and second communication device  110  are electrically connected to the same distribution switchboard  202  so as to carry out communications. Thus, also in the communication system that allows the communication devices to control the same device, the effects of communication device  100  and communication system  700  can be obtained. 
     The control methods of embodiments 1, 2, and 3 can be implemented by computer programs. The programs may be recorded in computer-readable recording media such as a CD-ROM or may be distributed through communication channels such as the Internet. 
     Moreover, communication standards used for communications in the embodiments are not particularly limited. For example, Wi-Fi (registered trademark), Bluetooth (registered trademark), BLE (Bluetooth (registered trademark) Low Energy), ANT (registered trademark), Zigbee (registered trademark), or original communication standards may be used. 
     As described above, the techniques of the present disclosure were illustrated as the embodiments. The accompanying drawings and detailed explanations were provided for the illustration. 
     Thus, the constituent elements described in the accompanying drawings and the detailed description may include the constituent elements not required for solving the problem but for illustrating the techniques as well as the constituent elements required for solving the problem. For this reason, the constituent elements described in the accompanying drawings and the detailed description should not be always regarded as being necessary for solving the problem. 
     The embodiments illustrate the techniques of the present disclosure and thus claims or the equivalent scope may involve modification, replacement, addition, and omission in various ways. For example, the present disclosure includes a configuration implemented by optionally combining the constituent elements and functions of the embodiments. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to a house controller for controlling household electrical appliances. The present disclosure can prevent the absence of communication devices having started DHCP server functions on a network if the communication devices having the DHCP server functions are present on the same network. Alternatively, the present disclosure is applicable to a house controller system including the multiple house controllers on the same network. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
         
           
               100 ,  100   a ,  100   b  communication device (first communication 
             device) 
               101 ,  101   a ,  101   b  communicator (first communicator) 
               102 ,  102   a ,  102   b  controller (first controller) 
               110 ,  110   a ,  110   b  second communication device 
               111 ,  111   a ,  111   b  second communicator 
               112 ,  112   a ,  112   b  second controller 
               200 ,  201 ,  202  distribution switchboard 
               300 ,  301  load device 
               400  network 
               410  external network 
               500  solar panel 
               600  system power supply 
               700 ,  700   a ,  700   b  communication system 
               800  fixed DHCP server 
               900  access point