What VPN and private network are ? - Part of ICT system

To understand what a virtual private network is, let's first think of what we mean by a private network. Many large organizations are geographically widespread. Think for instance of large chains of shops supermarkets or banks. Very often each shop, supermarket or bank will have its own LAN. Typically a large organization needs to join up its LANs for data communications. If the only data that needed to be transmitted were emails, then using the internet might be a satisfactory way to do this, however, large organizations have more complex data requirements than that. They may have servers holding files that need to be accessed by people throughout the organization, but which need to be inaccessible to people outside  the organization. Ideally, then the organization would have a private network which is a data network for the exclusive use of the organization.


Such a private network is separate from the internet, however, a large organization would almost need access to the internet from its own private network, the gateway is a potential vulnerability as far as keeping the network private  is concerned, but with good firewalls and other security the organization's own network should be essentially private.

Possible options for a private network with large geographical coverage could be:
- Lay down cables or fibres between outlying parts of the organization.
- Lease lines from a telecommunications company to join outlying parts of the organization.
- Create wireless links.
Any of these options is likely to be extremely expensive, and impractical for most organizations.

An alternative strategy is a virtual private network (VPN), which uses a shared or public network for part of its data transmission and yet, to its users, appears to be private. Very often the internet forms part of the shared network.

Several techniques exist for creating virtual private network, and usually they involve the internet protocol (IP). For the users of VPN the system is transparent , that is , to the VPN's users the servers and networked computers within the organizations are easily accessible from their own computers, even though they might be in another part of the country, or even in a different country. Data traffic between the user's own computer and the remote server may pass for part of the journey over the internet. However, other internet users outside the VPN are not able to see those servers from their computers. The network is therefore private, even though it uses shared data links.