Intermediary device with a first internet protocol-compatible connection side for connecting to at least one internal communication terminal and a second internet protocol-compatible connection side for connecting to an external communication network or an external communication line, wherein the intermediary device is embodied such that - on receipt of an internet protocol-compatible input data package, which originates from the at least one internal communication terminal and is addressed to an external internet protocol-compatible communication terminal which is connected to the external communication network, - it reads the internet protocol address of the internal communication terminal from the input data package, - it subjects the input data package to a protocol conversion, - it generates a protocol-converted output data package which carries the internet protocol address of the internal communication terminal as the internet protocol address of the transmit device, and - it sends the protocol converted output data package to the external communication terminal, and - on receipt of an internet protocol-compatible input data package, which originates from an external communication terminal which is connected to the second connection side and is addressed to the at least one internal communication terminal connected to the first connection side, - it reads the internet protocol address of the external communication terminal from the input data package, - it subjects the input data package to a protocol conversion, - it generates a protocol-converted output data package which carries the internet protocol address of the external communication device as the internet protocol address of the transmit device, and - sends the protocol-converted output data package to the internal communication terminal. - on receipt of an internet protocol-compatible input data package, which originates from the at least one internal communication terminal and is addressed to an external internet protocol-compatible communication terminal which is connected to the external communication network, - it reads the internet protocol address of the internal communication terminal from the input data package, - it subjects the input data package to a protocol conversion, - it generates a protocol-converted output data package which carries the internet protocol address of the internal communication terminal as the internet protocol address of the transmit device, and - it sends the protocol converted output data package to the external communication terminal, and - it reads the internet protocol address of the internal communication terminal from the input data package, - it subjects the input data package to a protocol conversion, - it generates a protocol-converted output data package which carries the internet protocol address of the internal communication terminal as the internet protocol address of the transmit device, and - it sends the protocol converted output data package to the external communication terminal, and - on receipt of an internet protocol-compatible input data package, which originates from an external communication terminal which is connected to the second connection side and is addressed to the at least one internal communication terminal connected to the first connection side, - it reads the internet protocol address of the external communication terminal from the input data package, - it subjects the input data package to a protocol conversion, - it generates a protocol-converted output data package which carries the internet protocol address of the external communication device as the internet protocol address of the transmit device, and - sends the protocol-converted output data package to the internal communication terminal. - it reads the internet protocol address of the external communication terminal from the input data package, - it subjects the input data package to a protocol conversion, - it generates a protocol-converted output data package which carries the internet protocol address of the external communication device as the internet protocol address of the transmit device, and - sends the protocol-converted output data package to the internal communication terminal.