VPN (Virtual Private Network) protocols are technologies used to establish secure and private connections over the internet. These protocols determine how data is transmitted, encrypted, and authenticated between the user and the VPN server. There are several types of VPN protocols, each with its strengths, weaknesses, and use cases. Below are the most commonly used VPN protocols:

1. PPTP (Point-to-Point Tunneling Protocol)
 - PPTP is one of the oldest and simplest VPN protocols. It was developed by Microsoft in the 1990s and is widely supported across many devices and platforms.

2. L2TP (Layer 2 Tunneling Protocol) / L2TP/IPsec
 - L2TP is a tunneling protocol that is often paired with IPsec (Internet Protocol Security) for encryption and security. L2TP itself does not provide encryption but, when combined with IPsec, offers a secure solution.

3. IPsec (Internet Protocol Security)
 - IPsec is a suite of protocols used to secure IP communications by encrypting data at the IP layer. It is often used in combination with other protocols like L2TP, IKEv2, and even PPTP to provide encryption and authentication.

4. OpenVPN
 - OpenVPN is an open-source VPN protocol that uses SSL/TLS for key exchange. It is highly customizable and can run on both TCP and UDP ports making it versatile in terms of performance and flexibility.

5. IKEv2 (Internet Key Exchange version 2)
 - IKEv2 is a modern VPN protocol that uses IPsec for encryption. It is known for its speed, stability, and ability to reconnect quickly when a network connection is lost (e.g., switching between Wi-Fi and mobile data).

6. SSTP (Secure Socket Tunneling Protocol)
 - SSTP is a VPN protocol developed by Microsoft that uses SSL/TLS over port 443, the same port used for HTTPS web traffic. This makes SSTP harder to block by firewalls and network filtering devices.

7. WireGuard
 - WireGuard is a modern, open-source VPN protocol that aims to be faster, simpler, and more efficient than traditional VPN protocols like OpenVPN and IPsec. It uses state-of-the-art cryptography and is designed to be easy to configure and audit.

8. SSL VPN (Secure Sockets Layer VPN)
 - SSL VPN is a category of VPNs that uses SSL/TLS protocols to secure communications. Unlike traditional VPNs, SSL VPNs typically allow user to access the network via a web browser without the need for special client software.