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A VPN is used to provide access to a private network over a public network. The public network is often the Internet, but it could also be leased lines that are shared by different companies. Figure 20.1 shows a common example of a how a VPN server is configured.
The VPN server has at least two network interface cards (NICs). One NIC has a public IP address and can be reached by any user who has access to the Internet. The other NIC has a private address connected to the internal network.