But when an organization had become larger, more and more members are out for businesses with third parties over the world. Many of them will need to access internal data from the outside. Some of them may have only an hour or so a day for accessing critical information, and most likely they will access to their LAN through internet.

Unlike LAN, internet is shared by users throughout the world. Everyone who access to the internet may gain access to data of everyone, and between them lie a tremendous amount of "unwelcome" third parties that want to steal information from others for good, or even control other computers for illegal use.

Fig 1.5 Third parties can use their computers to track the data flow between user and internal network

Here comes the problem: how to keep the data flow between the user computer and internal network secure from others?

The introduction of VPN

The terms VPN, which stands for Virtual Private Network, is introduced to provide a secure connection between remote computers to local area network (LAN) through internet to gain access to internal computer resources. Data between two ends are encrypted before transmission, making others impossible to interpret the content even they can track the data flow in the connection (Fig 1.6).

Fig 1.6 VPN connection between remote computer and VPN server

By using VPN, the communication in the internet can be protected by encryption. It provides a secure area for business and commercial information exchange. VPN also ensure integrity by checksum values, which reveals any attempt of modifying original data during the communication. Identity of the two ends will be confirmed by authentication, which makes sure that both side of the communication will be identified before the communication begins.

Some advantages of using VPN are:
- Cost saving on lease lines for remote users using public network. This is very important if many members of an organization need to connect outside the local area network
- Enable high speed remote user to securely connect though public network.
- No workload on monitoring direct access data links of remote users

Nowadays, the use of VPN has been widespread in different fields in the world. Some of the areas include:
- Banking (especially E-banking service)
- Some public service of the government
- Commercial (like E-commercial, E-shopping, etc.)
- Educational use