All of these overlays illustrate a concept that is central to computer networks in general: virtualization. That is, it is possible to build a virtual network from abstract (logical) resources on top of a physical network constructed from physical resources. Moreover, it is possible to stack these virtualized networks on top of each other and for multiple virtual network to coexist at the same level. Each virtual network, in turn, provides new capabilities that are of value to some set of users, applications, or higher-level networks.
The term virtualization is used a lot these days in the context of data center computing
to refer to the virtualization of servers using hypervisors and similar technologies but
it’s really a much broader concept.