A computer system is connectable to a network. The computer system includes a plurality of processing units, each of the processing units being provided with a respective network identity for communication with the network. At least one service processor is operable to allocate network identities to the processing units. A switch is provided for interconnecting the processing units to the network. The switch is operable to maintain a record of the network identities allocated to the processing units by the service processor and filters network access by each processing unit such that network access is blocked where a processing unit identity does not correspond to that held by the switch. By maintaining a record of the network identities allocated to the processing units by the service processor in the switch and filtering network access, access by a processing unit that has been changed or where its network identity has otherwise changed, can be prevented, maintaining the integrity of the network.