A switch 1 having a tag VLAN function includes a plurality of physical ports P1, P2 . . . , and adds an ID tag unique to each physical port to the header of a packet received by the physical port and sends the packet, and on the other hand, refers to an ID tag attached to a packet received from a GW 2 and sends the packet to a physical port corresponding to the ID tag. The GW 2 searches for a server address corresponding to the ID tag attached to the packet received from the switch 1 and modifies a destination address of the packet into the server address. In addition, the GW 2 attaches an ID tag corresponding to a source address of the packet received from the shared server 3 to the received packet, and modifies the source address of the packet into a GW address.