Which of the following is most commonly used in PPTP, L2TP/IPsec, SSTP, and OpenVPN?

The concept that is most commonly used in PPTP, L2TP/IPsec, SSTP, and OpenVPN is tunneling. Tunneling is a technique used to encapsulate a network protocol within packets carried by another protocol. This allows secure transmission of data over potentially insecure networks by creating a private "tunnel" through which data packets are sent.

In the context of the listed protocols:

• PPTP (Point-to-Point Tunneling Protocol) uses tunneling to encapsulate data transmitted over a network.

• L2TP (Layer 2 Tunneling Protocol) often combines with IPsec for encryption and secure tunneling.

• SSTP (Secure Socket Tunneling Protocol) leverages SSL (Secure Sockets Layer) for tunneling to provide a secure channel.

• OpenVPN is an open-source solution that facilitates the creation of secure point-to-point or site-to-site connections in routed or bridged configurations.

The essence of using tunneling in these protocols is to ensure secure communication between endpoints, effectively allowing them to send and receive data as if they were directly connected, even over the public internet. This feature is vital in establishing Virtual Private Networks (VPNs), which safeguard data integrity and privacy.

The other options