It is an interesting piece of history. This dates back from when MS thought that all networking should be peer-to-peer (and that 640K was a lot of memory). IBM

It is an interesting piece of history. This dates back from when MS thought that all networking should be peer-to-peer (and that 640K was a lot of memory). IBM knew better and built domain controllers into OS/2. As an anecdote - I worked on the team that was sub-contracted by IBM to write this. The spec was considered super secret and so had to be kept under lock and key. IBM security would confiscate any loose documents and leave a red card. One red card is a warning, two and you're off the project, three and the project is cancelled. We'd just had a red card so were on alert when, late one evening, we discovered an unlocked cupboard on the final sweep. So we saved the spec in another cuprboard and for a joke left the old red card behind. The PM, who's spec it was, came in early the next day - obviously before the devs who left late the night before, found the red card and phoned security to get it back. The ensuing "YOU'VE LOST THE SECRET SPEC!" conversation spiralled around like crazy until we came in and managed to calm everybody down. The PM still had to do a lot of grovelling. Moral of the story - probably best not to attempt a joke with anything security related.