A network-to-network bridge is provided. In one embodiment, the network-to-network bridge is coupled between a main system and a subsystem system. The main system includes a slot to couple with the subsystem, wherein the slot complies with the PCIe standard. The network-to-network bridge includes a transport layer and an internet layer but lacks of a network access layer allocated between the first main system and subsystem so as to transfer data by following the PCIe standard therebetween. The network-to-network bridge transfers data between the main system and the subsystem by accessing and employing their MAC addresses, and the network-to-network bridge can be allocated in the mainboard of the main system or the subsystem.