By following these steps, you should be able to download and install AutoCAD 2018 permanently.
If you are a business or professional, you must purchase a subscription from the Autodesk Store. Note that Autodesk has moved largely to subscription models; you may need to contact Autodesk Sales directly to inquire about accessing the 2018 legacy version, as they typically default to the latest release. autocad 2018 download and install permanently link