A coupling device for connecting one of two servers (6a, 6b) at a time to a data transmission network, wherein the coupling device comprises: - a first terminal for connecting a first server, - a second terminal for connecting a second server, - a third terminal for connection to a data transmission network, - a switch device comprising a first switching state and a second switching state, wherein - in the first switching state the third terminal is connected to the first terminal and disconnected from the second terminal, so that a first server connected to the first terminal is able to communicate via a data transmission network connected to the third terminal and a second server connected to the second terminal cannot communicate via the data transmission network, and - in the second switching state the third terminal is connected to the second terminal and disconnected from the first terminal, so that a second server connected to the second terminal is able to communicate via a data transmission network connected to the third terminal and a first server connected to the first terminal cannot communicate via the data transmission network, and - a power supply means (8; 14, 15) electrically connected to the switch device for supplying the switch device with energy, wherein the power supply means (8; 14, 15) is connected or can be connected to either a first server connected to the first terminal or a second server connected to the second terminal and is adapted to then receive energy from the respective server (6a, 6b) and provide an energy signal to the switch device, characterized in that the switch device is adapted to assume one of the first and second switching states when it is supplied with the energy signal by the power supply means (8; 14, 15), and to assume the other of the first and second switching states when it is not supplied with the energy signal by the power supply means (8; 14, 15); and in that the power supply means further comprises: - a power supply terminal for connection to a power supply cable of either a first server connected to the first terminal or a second server connected to the second terminal, and - a power supply line connecting the power supply terminal and the switch device to each other. - a first terminal for connecting a first server, - a second terminal for connecting a second server, - a third terminal for connection to a data transmission network, - a switch device comprising a first switching state and a second switching state, wherein - in the first switching state the third terminal is connected to the first terminal and disconnected from the second terminal, so that a first server connected to the first terminal is able to communicate via a data transmission network connected to the third terminal and a second server connected to the second terminal cannot communicate via the data transmission network, and - in the second switching state the third terminal is connected to the second terminal and disconnected from the first terminal, so that a second server connected to the second terminal is able to communicate via a data transmission network connected to the third terminal and a first server connected to the first terminal cannot communicate via the data transmission network, and - in the first switching state the third terminal is connected to the first terminal and disconnected from the second terminal, so that a first server connected to the first terminal is able to communicate via a data transmission network connected to the third terminal and a second server connected to the second terminal cannot communicate via the data transmission network, and - in the second switching state the third terminal is connected to the second terminal and disconnected from the first terminal, so that a second server connected to the second terminal is able to communicate via a data transmission network connected to the third terminal and a first server connected to the first terminal cannot communicate via the data transmission network, and - a power supply means (8; 14, 15) electrically connected to the switch device for supplying the switch device with energy, wherein the power supply means (8; 14, 15) is connected or can be connected to either a first server connected to the first terminal or a second server connected to the second terminal and is adapted to then receive energy from the respective server (6a, 6b) and provide an energy signal to the switch device, characterized in that the switch device is adapted to assume one of the first and second switching states when it is supplied with the energy signal by the power supply means (8; 14, 15), and to assume the other of the first and second switching states when it is not supplied with the energy signal by the power supply means (8; 14, 15); and in that the power supply means further comprises: - a power supply terminal for connection to a power supply cable of either a first server connected to the first terminal or a second server connected to the second terminal, and - a power supply line connecting the power supply terminal and the switch device to each other. - a power supply terminal for connection to a power supply cable of either a first server connected to the first terminal or a second server connected to the second terminal, and - a power supply line connecting the power supply terminal and the switch device to each other.