Joe,
If that's true, then why is the Internet not already a virtual net, e.g., supporting different transport protocols over IP? IP is shared by different transports. Similarly, ethernet supports different network protocols.In that sense, I'd view the Internet as supporting overlay networks, in the generic sense that an overlay network running (say) on end hosts could have packets traversing an overlay link be delivered via the Internet. But, the Internet does not support network virtualization in that you cannot instantiate a virtual network *inside* the Internet, running *in parallel to* (as opposed to *on top of*) IP.
-- Jen
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