A network is a virtual network that is built on top of another network. Links that are virtual or logical connect the nodes in the overlay network. Each link corresponds to a path, perhaps through many physical links, in the underlying network. The topology of the overlay network may (and often does) differ from that of the underlying one. For example, many peer-to-peer networks are overlay networks. They are organized as nodes in a virtual system of links that runs on top of the Internet. Overlay networks have been around since the invention of networking, when computer systems were connected over telephone lines using modems before any data network existed.
