1. A method for realizing a Virtual Network (VN), comprising: setting a plurality of virtual network access points (VN-APs) in a data center network, wherein some of the VN-APs are physical switches connecting a plurality of Personal Computers (PCs), wherein some of the VN-APs are virtual switches hosted by Personal Computers (PCs) connecting a plurality of Virtual Machines (VMs), and wherein each VN-AP in the plurality of VN-APs comprises a forwarding table for routing packets; receiving, by a service deployment and management function entity, a VN service request from a user to generate feature information of the VN, wherein the feature information of the VN includes the following information: a VN name, VN-ID of the VN, a MAC address of a PC in the PCs or a VM in the VMs (PC/VM) which can be connected with the VN, and a user name and a user password for authenticating VN's identity of the PC/VM; automatically discovering a VN-AP in the plurality of VN-APs by the PC/VM; and generating, by the automatically discovered VN-AP, a VN forwarding table entry of the PC/VM after the PC/VM passes VN's identity authentication, and performing tunnel encapsulation according to the VN forwarding table entry to forward a packet from the PC/VM, wherein the automatic discovery of the VN-AP by the PC/VM is: the PC/VM sends a VN-AP discovery message via an Ethernet frame of a specific type; all VN-APs in the plurality of VN-APs that are in a broadcast domain of the PC/VM return VN-AP provision messages to the PC/VM; the PC/VM selects one VN-AP from all the VN-APs which returned the VN-AP provision messages and sends a VN-AP request message to the selected VN-AP; and the selected VN-AP returns an acknowledgement/response message to the PC/VM, wherein the acknowledgement/response message includes VLAN identifier information or session identifier information used by the VN-AP and the PC/VM to separate the PC/VM from other PCs/VMs; wherein identity authentication comprises authenticating the identity of the PC/VM through Point to Point Protocol over Ethernet (PPPoE) or 802.1x protocol, and wherein after the PC/VM passes the identity authentication, an AAA server or the service deployment and management function entity returns a VN-ID to the discovered VN-AP; synchronizing the VN forwarding tables of all VN-APs of the VN through MP-BGP or extended MP-BGP, or a learning mechanism of a data plane.