<P> An overlay network is a virtual computer network that is built on top of another network . Nodes in the overlay network are connected by virtual or logical links . 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 of a virtual system of links that run on top of the Internet . </P> <P> 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 . </P> <P> The most striking example of an overlay network is the Internet itself . The Internet itself was initially built as an overlay on the telephone network . Even today, each Internet node can communicate with virtually any other through an underlying mesh of sub-networks of wildly different topologies and technologies . Address resolution and routing are the means that allow mapping of a fully connected IP overlay network to its underlying network . </P> <P> Another example of an overlay network is a distributed hash table, which maps keys to nodes in the network . In this case, the underlying network is an IP network, and the overlay network is a table (actually a map) indexed by keys . </P>

Why the internet is sometimes described as a network of networks
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