Right, and furthermore an address in 3.0.0.0/8 isn't likely to be added on any of your interfaces, so attempting to bind/listen on that will just fail. Even if you add it yourself, there's a tiny chance you might start redirecting some traffic you care about (say, facebook) to a local interface where facebook services are definitely not running. :-P

Fortunately, the whole 127.0.0.0/8 block is automatically added to the loopback interface (edit: on Linux at least), so you can bind away to your heart's content.

then how can you run the webapp if you aren't binding it to localhost?

How will your PC know how to resolve 3.0.0.0/8?