A network distributor has a plurality of input-output ports, a processor unit, and a memory unit. The ports connect network subscribers via a data line network, where the network subscribers include protocol subscribers to process telegrams as protocol telegrams. The processor unit receives telegrams via one port and outputs telegrams via another port, which is stored in a routing table in the memory unit. The processor unit further determines whether a telegram is a protocol telegram received via a port for which no port identifier is stored in the memory unit, and to discard the telegram if so. Checking and discarding of telegrams can be subject to a precondition, where fulfillment of the precondition leads to an exception from discarding the telegram.