Disclosed is a function-expandable wired network device using an external circuit to execute an operation an Ethernet device can't execute. The wired network device includes an Ethernet switch and a Field-Programmable Gate Array (FPGA). The switch includes Ethernet ports including a designated port and a first port, and receives a first packet from the first port; and if the first packet carries information meeting the information prestored in the switch, the switch amends the first packet to output a second packet to the designated port. The FPGA receives the second packet from the designated port and processes the second packet according to the switch's amendment to the second packet so as to output a third packet to the designated port. The switch then processes the third packet according to the FPGA's amendment to the third packet to output a fourth packet to one of the Ethernet ports.