Overview of VPN protocols.
VPNs (Virtual Private Networks) are often viewed from the perspective of security with the goal of providing authentication and confidentiality.
However, the primary purpose of VPNs is to connect 2 topologically separated private networks over a public network (typically the Internet).
VPNs basically hook a network logically into another network so that both appear as one private local network.
Security is a possible add-on to VPNs. In many cases it makes perfectly sense to secure the VPNs communication over the unsecure public network.
VPN protocols typically employ a tunnel where data packets of the local network are encapsulated in an outer protocol for transmission over the public network.
The most important VPN protocols are IPSec, PPTP and L2TP. In recent years SSL/TLS based VPNs such as OpenVPN have gained widespread adoption.