How can I access my nodejs web server from my local computer using the server domain name?

I installed nodejs and created a sample app. When I run npm start I get a message saying that I can open my web browser to http://localhost:3000 to see the app in action, but this installation is on a web server - not my local computer, so, instead of localhost:3000 I want to get there using something like mydomain.com:3000

I can't find the answer, it's very likely I just don't know how to search for it... any ideas?

I'm following the tutorial here: https://facebook.github.io/react/tutorial/tutorial.html

via andrux