A processing unit is connectable to a communications network. The processing unit has a data carrier reader operable to read a network identity from a portable data carrier. The processing unit is operable on being powered up to check for the presence of a said data carrier, and when a data carrier is present to use the network identity from the data carrier for communicating via the network. The data carrier can be in the form of a card having readable storage, for example a portable storage device having a readable semiconductor memory of a type know, for example, as a smart card, or memory card. The processing unit may be arranged to use only the network address read from the data carrier, and, in the absence of such a data carrier to prevent access to the network. Access to the network can be achieved by aborting the power up of the processing unit. The processing unit may therefore access the network only when the network address is present in the carrier reader when the processing unit is powered up.