Automatically join ansible inventory.

I have a bunch of linux machines, and I want to provision them all with the same disk image and have them automatically report their ip address somewhere that ansible can get them and add them to a dynamic inventory. I'm trying to figure out how to scale to 1000+ devices so I don't want to have to add them all by hand. The machines are physical machines, not cloud instances. The solution should use rely on free and open source software.