A virtual private network (VPN) is a private network that has been constructed over a shared, public infrastructure such as Frame Relay, an ATM network, or the Internet. It is considered virtual because it does not require a separate physical network, but instead it is a logical network, one of possibly many logical networks, that make use of a single physical network. It is considered a private network because a VPN can ha ve its own separate addressing and routing scheme to interconnect devices that need to communicate. A VPN is designed so that only devices supposed to communicate with each other can do so. For instance, a VPN can be the network infrastructure that provides communication between the corporate headquarters, branch offices, mobile users, suppliers, and customers, while ensuring that unwanted devices cannot gain access to this private network. Virtual Private Networks (VPNs) is not a new concept for the European Research & Education community. It is used since a long time to support testbeds, pilot services (e.g. Mbone, 6Bone) and other European projects. It took several forms so far
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