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Autodesk maintains a strict product lifecycle policy. The company supports the current version of AutoCAD plus up to three or four previous versions. Because AutoCAD 2006 is decades old, Autodesk closed the activation servers for this version years ago.
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Understanding how legacy software like AutoCAD 2006 handles licensing requires a look into the history of digital rights management (DRM). When AutoCAD 2006 was released, Autodesk utilized an activation system driven by two distinct identifiers: a Request Code and an Activation Code.