A VPN (Virtual Private Network) [I1] [I2] [F1] [F2] [S1] [S2] [E1] is an hardware and/or software technique to create a private net of an organization by using transmission channels and shared connections (internet).
You can make these connections safer by using cryptography algorithms of symmetric and asymmetric type. In this way the information travelling on the VPN is coded so that anybody can take it but nobody can use it. This concept is realized using the idea of the safe tunnel. The main idea is to build a tunnel between my local net and the opposite local net. In this net those who want to steal our secrets are not able to do it unless our keys are no longer secret which means that we have not been very careful.
To give you an example of the different ways to use this mechanism you can think of an IP packet.
As you know an IP packet is made of one part that brings data that we want to make safe. It is possible to code these data (this packet) with a secret key to sent it completely coded. It is kept coded along the whole channel so that nobody can read the information. This technique is said IPSec which means that the IP protocol is safe.
