To provide a relay server which is dynamically adaptive to change in registration state of a private network group due to movement of a communication terminal etc.

The relay server is disposed in a LAN. Account information of a communication terminal in the LAN and group information regarding the private network group are registered. An external server is used to establish a communication session with another relay server. When no group information is held, a request to transmit group information is sent to the other relay server. When the group information that the other relay server holds is changed, a request to transmit the group information is made. When transmitting and receiving communication terminals are in the same network group, communication data between the communication terminals are relayed through a communication session for relaying.

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