Computer Networks : Principles, Technologies And Protocols For Network Design By Natalia Olifer And Victor Olifer

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Material in this section is based on material from Computer Networks: Principles, Technologies and Protocols for Network Design by Natalia Olifer and Victor Olifer (pub. John Wiley & Sons 2005). The term ‘VPN’ has no standard interpretation. Different networking specialists and different organisations may understand it in different ways. Historically, the term was first introduced by telephone companies. The main feature of a telephone VPN is that it can provide users from an organisation which uses a public provider’s telephone service instead of its own private PBX with something very close to PBX functionality (commonly known as Centrex, and popular in North America). For example, they can dial using convenient private (usually short) numbers; certain phones can be fully or partly isolated from the public telephone network; and users can use PBX-style telephone services like call forwarding, call rerouting, voice mail etc. For data networks the term came to be used later, at first mainly for services which provide more security than the standard Internet service due to user data encryption. However, there are also services that do not encrypt user data but create logical channels for users within public data networks and provide controllable connectivity between VPN users and with the outside world. (Some of these elements were available to the data networking community in the closed user group of X.25 and the filtering capabilities of SMDS in previous incarnations of

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