According to one embodiment, there is provided a communication system includes endpoints and server units. The endpoints include a transmitter and an information assignment module. The transmitter transmits a message of the registration request to one of the server units. The information assignment module assigns information used to discriminate a cause of generation of the registration request to the message. The server units include a specification module, a determination module and a registration processor. The specification module specifies the cause based on the information assigned to the received message. The determination module determines whether or not permit registration of the endpoint as a source of the received message based on the specified cause and priority level. The registration processor registers the endpoint when the registration is permitted. The registration processor instructs the endpoint to redirect the message when the registration is not permitted.