VPN

A virtual private network (VPN) is a computer network in which some of the links between nodes are carried by open connections or virtual circuits in some larger networks, such as the Internet, as opposed to running across a single private network. The Link Layer protocols of the virtual network are said to be tunneled through the transport network. One common application is to secure communications through the public Internet, but a VPN does not need to have explicit security features such as authentication or content encryption. For example, VPNs can also be used to separate the traffic of different user communities over an underlying network with strong security features, or to provide access to a network via customized or private routing mechanisms.
VPN service providers may offer best-effort performance, or may have a defined service level agreement (SLA) with their VPN customers. Generally, a VPN has a topology more complex than point-to-point.