Short for Virtual Private Network, a VPN is a network in which some of the nodes are connected using the public Internet, but the data sent across the Internet is encrypted, so the entire network is virtually private. Virtual private networks are commonly used by businesses that have offices in several cities, and would like to ensure that information passed from office to office over the public internet links can not be intercepted and viewed by third parties. Hence it is as if the company is operating a secure intranet.

Properly installed VPNs use cryptographic tunneling protocols to provide the necessary confidentiality (preventing snooping), sender authentication (preventing identity spoofing), and message integrity.