A network processor system includes a network processor 1, which is provided with a number of interfaces 9, 11, 13, 15, 19, including one or more Ethernet interfaces 9, 11 and a UART interface 19, which can be used to load a software package into the network processor 1. The software package can thus be modified when required, or when the software package previously stored in the network processor system is corrupted. The Ethernet data packets received on the Ethernet interfaces 9, 11 are in a special format, which the network processor 1 can recognize, to distinguish them from conventional Ethernet packets. Thus, the network processor 1 does not have to be programmed to handle the full range conventional Ethernet data packets. The network processor 1 is provided with a second external non-volatile data storage device 7 in the form of a EEPROM memory 7. The EEPROM memory 7 provides a writable extension of the internal ROM memory of the network processor. It can, for example, be used to store the parameters of other components of a network processor system (e.g. a flash memory 5) so that the network processor 1 can be used in conjunction with components of a wide range of types.