1. A server, comprising: a virtual machine identifier assigning section to assign an identifier of a virtual machine operating on the server; and a network interface to transmit a packet including a Layer 2 header information which includes the identifier of the virtual machine and a first packet field for a VLAN-Tag, wherein the network interface transmits the packet to a packet encapsulate section which encapsulates a second packet field including the Layer 2 header information with a virtual network identifier representing a virtual network to which the virtual machine belongs.