Ah, that’s a completely different situation.
If you are writing something for a closed group of users rather than a public website you can code according to the known without worrying about web best practice.
For example, if everyone in the office is using IE11 with 1800x900 monitors or the same model and version of Android mobile, you can take a lot of “short cuts”.
You also will not need to consider the folly of trying to detect devices as you already know them.
IMHO much better to just go with something like “whatever.com” and “mobile.whatever.com”