Are thread ID numbers unique across a site?

I am developing a database to keep track of threads and I am using their thread Id number as their reference.

I see that that a thread is identified at the end of a url as such:


Where 3377 is the number of the thread. What I need to know, please, is this number going to be unique site wide? Or can two different threads have the same identifier number from two different forums of a site?