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Computer networks are typically a shared resource used by many applications for many different purposes. Sometimes the data transmitted between application processes is confidential, and the application users would prefer that others not be able to read it. A firewall is a specially programmed router that sits between a site and the rest of the network. It is a router in the sense that it is connected to two or more physical networks and it forwards packets from one network to another, but it also filters the packets that flow through it. A VPN is an example of providing a controlled connectivity over a public network such as the Internet. VPNs utilize a concept called an IP tunnel-a virtual point-to-point link between a pair of nodes that are actually separated by an arbitrary number of networks. In this paper we set up a network where servers are accessed over the Internet by customers who have different privileges. We study …