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overlay networkA logical network that runs on top of another network. For example, peer-to-peer networks are overlay networks on the Internet. They use their own addressing system to determine how files are distributed and accessed, thus providing a layer on top of the Internet's IP addressing.
A quality of service (QoS) system is also called an overlay network, because it uses an additional addressing mechanism that may change the route normally taken in the underlying network. See also over-the-top broadcasting and over-the-top messaging.