A system unit has at least a first network identity and a second network identity for communicating with a network, and an indication of which of the network identities was effective on the most recent complete initialization of the system unit. On a subsequent initialization of the system unit, for example after moving the system unit, the effective network identity is changed. The indication is reset to the effective network identity if initialization completes. A system unit can thus be taken out of service and then restarted with a different identity. This enables the system unit reliably to check information from the network without fear of conflict with its previous network identity. The system unit can be a service processor that is operable to allocate network identities to further units (e.g. on a shelf on which the service processor resides). The service processor can thus check on initialization that network identities that it is to allocate are not know already on the network, for example if the service processor had already allocated those identities to system units at a location from which it had been moved, to avoid conflicts occurring.