如何找到运行rails实例的本地端口？

So I would like my Rails app instances to register themselves on a "I'm up" kind of thing I'm playing with, and I'd like it to be able to mention what local port it's running on. I can't seem to find how to do it - in fact just finding out its IP is tricky and needs a bit of a hack.So I would like my Rails app instances to regis