It's more that `CORP` was the default short name for the AD domain, and also Windows ticks the "try superdomains" box in search domains by default. So if you h

It's more that `CORP` was the default short name for the AD domain, and also Windows ticks the "try superdomains" box in search domains by default. So if you have your fully-qualified AD domain set to `ad.example.com`, that's the default search domain too, and a DNS lookup for `corp` will first check `corp.ad.example.com` then `corp.example.com` then `corp.com`. Now, if you're using the AD server as your DNS server then it shouldn't get that far -- `corp.ad.example.com` should resolve. But if for whatever reason a device connected to AD doesn't use the AD server as its DNS server, for example if it's a laptop and not connected to the corporate network, then you'll be offered a _different_ search domain like `myisp.com`. Which probably _won't_ resolve `corp.myisp.com`. So the built-in resolver will after all walk its way up the search domain and check `corp.com` QED.