An
overlay network is a
computer network that is built on top of another network.
Nodes in the overlay network can be thought of as being connected by virtual or logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network. For example,
distributed systems such as
peer-to-peer networks and
client-server applications are overlay networks because their nodes run on top of the
Internet. The Internet was originally built as an overlay upon the
telephone network, while today (through the advent of
VoIP), the telephone network is increasingly turning into an overlay network built on top of the Internet.