A processing unit, for example a computer server, that is connectable to a data communications network, has a device reader for reading a supplied network identity from a portable storage device such as a smart card or the like. The processing unit then uses the supplied network identity from the portable storage device for communicating via the data communications network. The processing unit monitors the continued presence of the portable storage device. In the event that the processing unit detects that the portable storage device has been removed from the device reader, it signals a fault state. The processing unit can be arranged to power itself down where a portable storage device having same network identity is not returned in the device reader within a predetermined time.