The invention relates to a network interface apparatus called a router or the like which is provided in a network system and relays the transmission of a data packet as a bundle of information. In order to know the permission or inhibition of communication of the data packet at a high speed while suppressing an increase in memory capacity, there is provided a coupling management table in which one VLAN ID is allocated to each terminal (each MAC address) and coupling information showing whether the communication from the terminal having a transmitting side VLAN ID to the terminal having a receiving side VLAN ID is permitted (logic “1”) or inhibited (logic “0”) has been stored at each of the coordinate points using the VLAN ID on the transmitting side and the VLAN ID on the receiving side as coordinates.