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Computing (FOLDOC) dictionary
Virtual Private Network
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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
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.