V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

VPN
       Virtual Private Network


The Free On-line Dictionary of Computing (30 December 2018):

Virtual Private Network
VPN

    (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)