vpn

from The Free On-line Dictionary of Computing (8 July 2008)
Virtual Private Network
VPN

   <networking, security> (VPN) The use of {encryption} 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 system at both
   ends.  The encryption may be performed by {firewall} software
   or possibly by {routers}.

   Link-level (layer 2 and 3) encryption provides extra
   protection by encrypting all of each {datagram} except the
   link-level information.  This prevents a listener from
   obtaining information about network structure.  While
   link-level encryption prevents traffic analysis (a form of
   attack), it must encrypt/decrypt on every {hop} and every
   path.

   Protocol-level encryption (layer 3 and 4) encryption encrypts
   protocol data but leaves protocol and link headers clear.
   While protocol-level encryption requires you to
   encrypt/decrypt data only once, and it encrypts/decrypts only
   those sessions that need it, headers are sent as clear text,
   allowing traffic analysis.

   Application (layer 5 up) encryption is based on a particular
   application and requires that the application be modified to
   incorporate encryption.

   Cisco
   
(http://cisco.com/univercd/cc/td/doc/product/lan/cat5000/cnfg_nts/rsm/rsm_pa/4801encr.htm).

   (1999-11-15)