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Definition
VPNs have emerged as the key technology for achieving security over the Internet. While a VPN is an inherently simple concept, early VPN solutions were geared towards large organizations and their implementation required extensive technical expertise. As a consequence, small and medium-sized businesses were left out of the e-revolution. Recently, VPN solutions have become available that focus specifically on the needs of small and medium-sized businesses. Historically, the term VPN has also been used in contexts other than the Internet, such as in the public
telephone network and in the Frame Relay network. In the early days of the Internet-based VPNs, they were
sometimes described as Internet-VPNs or IP-VPNs. However, that usage is archaic and VPNs are now synonymous
with Internet-VPNs.

Overview and Benefits
A firewall is an important security feature for Internet users. A firewall prevents data from leaving and entering an enterprise by unauthorized users. However, when packets pass through the firewall to the Internet, sensitive data such as user names, passwords, account numbers, financial and personal medical information, server addresses, etc. is visible to hackers and to potential e-criminals. Firewalls do not protect from threats within the Internet. This is where a VPN comes into play.

A VPN, at its core, is a fairly simple concept-the ability to use the shared, public Internet in a secure manner as if it were a private network. the flow of data between two users over the Internet when not using a VPN. As shown by the dotted lines, packets between a pair of users may go over networks run by many ISPs and may take different
paths. The structure of the Internet and the different paths taken by packets are transparent to the two users. With a VPN, users encrypt their data and their identities to prevent unauthorized people or computers from looking at the data or from tampering with the data.

Applications
A VPN can be used for just about any intranet and e-business (extranet) application. Examples on the following pages illustrate the use and benefits of VPN for mobile users and for remote access to enterpriseresources, for communications between remote offices and headquarters, and for extranet/e-business.

Remote Access

In this application, when not using a VPN, mobile and remote users often use analog (dial-up modems) or ISDN switched services to connect to a headquarters data center. This is shown in figure 2a. These connections are used to access e-mail, to download files and to execute other transactions. This type of connection would also be used by small offices that do not have a permanent connection to the enterprise intranet.

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