A network relay device for relaying communication for a regular terminal via a port includes an acquiring module, a regular terminal information storing module, and a determination process module. The acquiring module acquires a regular layer 2 address, a regular layer 3 address, regular VLAN information representing a VLAN assigned to the regular terminal, and regular port information representing a port to which the regular terminal is connected. The regular terminal information storing module stores regular terminal information representing a combination of the acquired regular layer 2 address, the regular layer 3 address, the regular VLAN information, and the regular port information. The determination process module determines whether the combination of source layer 2 address, source layer 3 address, assigned VLAN, and reception port of target frame data received via the port is stored as the regular terminal information.