1. A server, comprising: a memory storing instructions; and a processor configured to execute the instructions to: assign an identifier of a virtual machine operating on the server; 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; and encapsulate the packet, to generate 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.