How to address students in the USA, as a university instructor: first name or last name? It is often asked whether students should be encouraged to address their instructors by their first or last name. My question is the opposite.

I teach at a large US university but I have not grown up here. Most students address me by my last name or by 'Professor', which seems to be the traditional way to do it. 

Questions:
It turn, would I traditionally address students by their first name or their last name? Or just "student"? Is the communication symmetrical or asymmetrical? Is there a difference in email and personal interaction?
First name, at every university I've been. Both for in person communication and email. There are a few charmingly old fashioned professors who use Mr/Miss Lastname, but they're seen as oddities. So yes, the communication is generally asymmetrical.