Virtual Private Network (VPN) has two different meanings:
The conventional VPN refers to a virtual private (self-contained) communications network. Virtual in the sense that it is not a separate physical connection, but an existing communications network that is used as a transport medium. The VPN is used to connect participants of the existing communication network to another network.
For example, an employee's computer can gain access to the company network from home, just as if he were sitting in the middle of it. From the point of view of the VPN connection, the intervening networks (his home network as well as the Internet) are reduced to the function of an extension cable that connects the computer (VPN partner) exclusively to the assigned network (VPN gateway). He now becomes a part of this network and has direct access to it. The effect is comparable to plugging the computer network cable to the network assigned by VPN.
This process works regardless of the physical topology and network protocols used, even if the assigned network is of a completely different type.
Depending on the VPN protocol used, the resulting benefits of a VPN can be supplemented by encryption, which enables tap-proof and tamper-proof communication between the VPN partners. Establishing an encrypted (virtual) network over an unencrypted network can be an important criterion, sometimes even the main reason for using a VPN.
Since 2002, SSL VPN (also known as web-based VPN) has supported solutions that realize encrypted remote access to corporate applications and shared resources without the SSL VPN partners having to bind themselves to the corporate network for this purpose. In this case, the network cable is not connected to another network; only secure access to certain services of the other network is enabled.
The name "VPN" for these solutions is controversial, but common on the market. Technically, they are based on a proxy mechanism (Thin Client SSL VPN) or on the fact that the coveted enterprise application is itself a web application (Clientless SSL VPN), which an SSL VPN partner can access via a secure connection, but without gaining direct access to the corporate network. In addition, SSL VPN also supports a VPN mode in the sense of conventional VPN (Fat Client SSL VPN).