Network Sales and Services Handbook (Cisco Press Networking Technology)

A virtual private network (VPN) is a public network being used for private communication. The VPN connection is an authenticated and encrypted communications channel, or tunnel, across this public network, such as the Internet. Because the network is considered insecure, encryption and authentication are used to protect data while in transit. VPN service is considered to be independent, in that client operation (for example, encryption) is transparent to the user and that all information exchanged between the two hosts World Wide Web [WWW], File Transfer Protocol [FTP], e-mail, and so on) is transmitted across the encrypted channel.