Track player location/sublocation

I am trying to make an MMO and trying to work through some of the problems I will run into. This question might have already been answered but I am not even sure how to phrase it really, so if anyone knows what this problem is called I'd love to know.

I will have a map with locations (or zones) and even more sublocations to be able to tell other players where you are (i want a coordinate system too but thats another day). And this is sort of a 2 part questions.
1. How can I keep track of where the player is? Example, I know the player is in location/sublocation lets just say "AA/AA" and he has crossed an invisible sublocation boundary into "AA/BB" i want "BB" to show up on the screen and next to a minimap it will now show "AA/BB" instead.
2. How can I do these invisible boundaries, and also how to keep them from being square as well?

Please note, I am only interested in general ideas, for example I don't need to know how to put it next to my minimap or how to have it show up on my screen, just how to know exactly where they are.
I like to figure out as much as I can on my own.
Thank you everyone in advance.