20.6 Virtual Private Networks and IPSec VPN - the use of encryption and authentication in the lower protocol layers to provide a secure connection through an otherwise insecure network, typically the Internet. VPNs are generally cheaper than real private networks using private lines but rely on having the same encryption and authentication system at both ends. The encryption may be performed by firewall software or possibly by routers. Applications of IPSec - it provides the capability to secure communcations across a LAN, across private and public WANs, and across the Internet. A key exchange function.
Transport mode provides protection primarily for upper-layer protocols.
Tunnel mode provides protection to the entire IP packet. Used when one or both ends is a security gateway, such as a firewall or router that implements IPSec. IPSecs give managers a standardized means of implementing security for VPNs. They can be implemented in routers or firewalls owned and operated by the organization.