It's not really about the structure of the URL it's there's not a good way for someone to know what URL goes with an IRL identity.
What's the URL for Valve's Steam?
* steam.com
* valve.com
* steamgames.net
* store.steampowdered.com
* steampowered.com
If you guessed none of them except for steampowered.com, congrats!
What's the URL for Americal Eagle, the clothing store?
* americaneagle.com
* americaneagleoutfitters.com
* aeo.com
* ae.com
* aerie.com
If you guesses all of them except americaneagle.com congrats!