A sub-system for forming part of a packet-data communication system for the transmission of data packets which contain media access control addresses and protocol addresses. The sub-system comprises a multiple-port network switch for forwarding data packets in response to an examination of the packets on the basis of the packets protocol addresses, and for performing said examination when a destination address in the packet corresponds to the media access control address of the switch, the switch being arranged for forwarding a packet on the basis of a media access control address therein when the destination address of the packet does not correspond to the media access control address of the switch, said switch including a forwarding database which comprises entries which relate a protocol address with a media access control address and an identification of a particular port of the switch. This switch may perform routing within a local-area network. The switch is configured so that protocol addresses may not be learned in the database in respect of the particular port. A network router, which may perform routing functions in respect of a wide-area network, is connected to the switch only by way of the particular port. The sub-system enables the switch to off-load from the router routing functions for the local-area network.