Overlay network

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An overlay network is a computer network that is layered on top of another (logical as opposed to physical) network. The concept of overlay networking is distinct from the traditional model of OSI layered networks, and almost always assumes that the underlay network is an IP network of some kind. ^[1]

Some examples of overlay networking technologies are, VXLAN, BGP VPNs, both Layer 2 and Layer 3, and IP over IP technologies, such as GRE or IPSEC Tunnels. IP over IP technologies, such as SD-WAN are a class of overlay network.