Quick Answer: A virtual private network is an overlay network that uses network virtualization to extend a private network across a public network, such as the Internet, via the use of encryption and tunneling protocols. In a VPN, a tunneling protocol is used to transfer network messages from one network host to another. Host-to-network VPNs are commonly used by organizations to allow off-site users secure access to an office network over the Internet. Site-to-site VPNs connect two networks, such as an office network and a datacenter. Provider-provisioned VPNs isolate parts of the provider's own network infrastructure in virtual segments, in ways that make the contents of each segment private with respect to the others. Individuals also use VPNs to encrypt and attempt to anonymize their network traffic, with VPN services selling access to their own private networks.