simply put, the fast travelling map is divided up into safe zones and hostile zones
if you're in a safe zone (which you can check by flicking a switch on the map which turns on a colour coded or stylized indicator) you can travel safely to any other location you've found within that safe zone. for example, the whole of the main city of new vegas is pretty much a safe zone as you don't encounter any creatures there. you can't fast travel if it involves passing through a zone the game deems hostile, and you can't fast travel if you're in a hostile zone. simples
this is pretty much how i play the game anyway!