To separate a part of network processing from the user environment.

When the communication system at the datalink level of a real network is Ethernet (R), the processing for making the MAC address of a real NIC identical to the MAC address of a virtual NIC is automated. When the real network is not Ethernet, data link communication of the real network is changed to communication by Ethernet for the virtual NIC of user VM. In this case, proxy ARP is performed for the virtual NIC of user VM. With regard to the IP address information for the virtual NIC of user VM, the IP address information of the real network is reused. With regard to communication from the virtual NIC of user VM, a processing method is determined from an Ethernet frame and its payload. With regard to communication from the real network, a processing method is determined from an Ethernet frame or an IP packet.