A network device including an Ethernet transmission interface and a processing unit is provided. The Ethernet transmission interface is provided with at least one Ethernet transmission port. The processing unit is coupled to the Ethernet transmission interface and is configured to: in order to detect whether a packet looping exists, send out a dynamic host configuration protocol (DHCP) discover message through a linked Ethernet transmission port in response to linking one of the at least one Ethernet transmission port to a network and determine whether a DHCP offer message is received; determine whether to prohibit data transmission of the linked Ethernet transmission port according to whether the DHCP discover message returned through the packet looping is detected; classify the linked Ethernet transmission port as an uplink transmission port or a downlink transmission port according to whether the DHCP offer message is received.