Virtual Private Networks or VPNs are called "virtual" because they form temporary connections that have no real physical presence. They consist of packets routed over various machines on an as-needed basis. VPNs make use of public connections, such as the Internet, to create secure private networks. VPNs might be used to connect two different company sites by means of the Internet, for example, or to connect a remote user to a site.
VPNs are a more cost-effective means of point-to-point secure communication than the use of dedicated secure phone lines. Before the VPN protocol, expensive lines were dedicated between users, or companies, to minimize access to others. The VPN protocol achieves the same level of security over public line (the Internet) using enhanced encryption techniques.
The security techniques involved include encryption, authentication, and firewalls--and an additional concept, packet tunneling. Packet tunneling enables encapsulation of one data packet within another data packet (or of one IP packet within another IP packet) to accommodate incompatible protocols. The following are some commonly used tunneling protocols:

1. AltaVista Tunnel
2. Point-to-Point Tunneling Protocol (PPTP)
3. Layer 2 Forwarding Protocol
4. IP Security (IPSec) tunnel mode

VPN Security Techniques
The next lesson wraps up this module.