8.1 What Is a VPN?

Consider a company that has a set of geographically dispersed sites. To interconnect computers at these sites, the company needs a network. The network is private in the sense that it is expected to be used by only that company. It is also private in the sense that the routing and addressing plan within the network is completely independent of the routing and addressing plans of all other networks. The network is virtual in the sense that the facilities used to build and operate such a network may not be dedicated just to that company, but could be shared with other companies that want to have their own VPNs. The facilities needed to build such a network are provided, at least partially, by a third party, known as a VPN service provider. The company that uses the network is known as a VPN customer.

Informally, we could say that a VPN is a set of sites that can communicate with each other. More formally, a VPN is defined by a set of administrative policies that control both connectivity and Quality of Service among sites.

It is quite natural that it is the VPN customer who sets up the policies that define that customer s VPN. With respect to who implements these policies, depending on the technology used, there could be a range of choices. For example, it should be possible to completely confine the implementation of these policies to the VPN service provider, so that the...