Virtual Private Networks (VPNs) are networks implemented such that small, not necessarily contiguous, sections of the network appear to users to constitute a complete, isolated, network. For example, a consultant, might build a VPN consisting of a home PC, their office PC(s) and PCs at several client offices. A school might establish multiple VPNs -- one for teachers, another for students and teachers, and a third for teachers and parents. Even though each of the VPN computers might be on a different physical network and connected to the Internet communications backbone by different technologies, a user at any of the participant PCs would see just the small, apparently tightly coupled, private network.

A real private network is coupled by dedicated private lines. A virtual private network is coupled by encrypted data sent over public data paths. On the positive side, VPNs allow users anywhere that has internet (or other global network) access to connect to servers or peers anywhere on the net. Further access is controlled by the sever/peer as if the users were local. In almost all cases, the connections will be made by low cost local telephone calls or dedicated broadband connections rather than a higher tariff longer distance phone line. The downside is that VPNs provide a tunnel through which intruders can penetrate firewalls if any defects exist in the VPN setup.

The major technical problems with VPNs are the issues of establishing and maintaining secure "private" links through a public message space. A variety of technologies have been proposed for VPNs. Several commercial products are available as of Q3 2001.



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