VPN uses encryption to securely connect private networks to the public internet. Key reflexive encryption uses unique identification numbers to encrypt specific applications. Link encryption devices encrypt data on a bit-by-bit basis when transmitting messages over communication lines. Firewalls control network access and communication to maintain internal network security.

5. Fire wall The software that was aimed at controlled a specific computer network and communication with the outside, and maintaining the security of the internal computer network.

6. Reference http;//www.google.co.jp/patents?hl=en&lr=&vid=USPAT5768271&id=MUOmAAAAEB&oi=fnd&dq= virtual+private+network&printsec=abstract#v=onepage&q=&f=false http://ja.wikipedia.org/wiki/%E3%83%95%E3%82%A1%E3%82%A4%E3%82%A2%E3%83%BC% E3%82%A6%E3%82%A9%E3%83%BC%E3%83%AB http://www.google.co.jp/patents?hl=ja&lr=&vid=USPAT5636280&id=mEsfAAAABAJ&oi=fnd&dq= Encryption+Security+&printsec=abstract#v=onepage&q=&f=false http://www.google.co.jp/patents?hl=ja&lr=&vid=USPAT5636280&id=mEsfAAAAE BAJ&oi=fnd&dq=Encryption+Security+&printsec=abstract#v=onepage&q=&f=false