1. VPN Devices
   • Function: Hardware or software used to create secure tunnels for encrypted communication over networks.
2. IPsec Compatible VPNs
   • Encryption Modes:
     • Tunnel Mode: Encrypts the entire data package, including the IP header.
     • Transport Mode: Encrypts only the datagram, leaving the original IP header unencrypted.
   • OSI Layer: Works at the Network layer of the OSI model.
   • Use Case: Provides secure IP-based communication.
3. Non-IPsec Compatible VPNs
   • Socks-based Proxy Servers:
     • Function: Allows access to the internal network from the outside.
     • Security: Contains strong encryption and authentication methods.
   • PPTP (Point-to-Point Tunneling Protocol):
     • Function: Commonly used in Windows machines for creating VPN connections.
     • Authentication: Uses PAP or CHAP for secure authentication.
   • Dial-up VPNs:
     • Function: Utilizes remote access servers with PPTP, often used by ISPs to provide secure access.
   • Secure Shell (SSH2):
     • Function: Opens a secure, encrypted shell session from the internet through a firewall to an SSH server.
     • Note: SSH2 is not strictly a VPN product but offers similar secure communication capabilities.

These VPN devices and protocols help create secure communication channels over public and private networks, ensuring that data remains confidential and protected during transmission.