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A virtual private network (VPN) extends a private network across a public network, such as the Internet. It enables users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network, and thus are benefiting from the functionality, security and management policies of the private network. A VPN is created by establishing a virtual point-to-point connection through the use of dedicated connections, virtual tunneling protocols, or traffic encryption. A VPN spanning the Internet is similar to a wide area network (WAN). From a user perspective, the extended network resources are accessed in the same way as resources available within the private network.〔(【引用サイトリンク】title=Virtual Private Networking: An Overview )〕 Traditional VPNs are characterized by a point-to-point topology, and they do not tend to support or connect broadcast domains. Therefore, communication, software, and networking, which are based on OSI layer 2 and broadcast packets, such as NetBIOS used in Windows networking, may not be fully supported or work exactly as they would on a local area network (LAN). VPN variants, such as Virtual Private LAN Service (VPLS), and layer 2 tunneling protocols, are designed to overcome this limitation. VPNs allow employees to securely access the corporate intranet while traveling outside the office. Similarly, VPNs securely connect geographically separated offices of an organization, creating one cohesive network. VPN technology is also used by individual Internet users to secure their wireless transactions, to circumvent geo-restrictions and censorship, and to connect to proxy servers for the purpose of protecting personal identity and location. ==Types== Early data networks allowed VPN-style remote connectivity through dial-up modems or through leased line connections utilizing Frame Relay and Asynchronous Transfer Mode (ATM) virtual circuits, provisioned through a network owned and operated by telecommunication carriers. These networks are not considered true VPNs because they passively secure the data being transmitted by the creation of logical data streams.〔Cisco Systems, et al. ''Internet working Technologies Handbook, Third Edition''. Cisco Press, 2000, p. 232.〕 They have been replaced by VPNs based on IP and IP/Multiprotocol Label Switching (MPLS) Networks, due to significant cost-reductions and increased bandwidth〔Lewis, Mark. ''Comparing, Designing. And Deploying VPNs''. Cisco Press, 2006, p. 5〕 provided by new technologies such as Digital Subscriber Line (DSL)〔International Engineering Consortium. ''Digital Subscriber Line 2001''. Intl. Engineering Consortium, 2001, p. 40.〕 and fiber-optic networks. VPNs can be either remote-access (connecting a computer to a network) or site-to-site (connecting two networks). In a corporate setting, remote-access VPNs allow employees to access their company's intranet from home or while traveling outside the office, and site-to-site VPNs allow employees in geographically disparate offices to share one cohesive virtual network. A VPN can also be used to interconnect two similar networks over a dissimilar middle network; for example, two IPv6 networks over an IPv4 network.〔(【引用サイトリンク】title=IPv6 traffic over VPN connections )〕 VPN systems may be classified by: * The protocols used to tunnel the traffic * The tunnel's termination point location, e.g., on the customer edge or network-provider edge * Whether they offer site-to-site or network-to-network connectivity * The levels of security provided * The OSI layer they present to the connecting network, such as Layer 2 circuits or Layer 3 network connectivity 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「virtual private network」の詳細全文を読む スポンサード リンク
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