A coupling device for connection of one of two servers at a time to a data transmission network as well as to a data transmission network having a coupling device. The device has a first and second terminal for connecting a first and second server and a third terminal for connection to the data transmission network. A switch device is provided having a first switching state where the third terminal is connected to the first terminal and disconnected from the second terminal, and a second switching state, where the third terminal is connected to the second terminal and disconnected from the first terminal. The switch device is adapted to assume a switching state, when it is provided with the energy signal by the power supply, and to assume another switching state, when it is not provided with the energy signal by the power supply.