So, I made a cool demo which I want to show to my friends. But when I give them my IP-address (with the Development server's port and aspx page), it doesn't connect. How can I make it work?
They can't see the development server port. It only serves pages to browser requests on the local computer.
You have to publish it to your IIS or to another machine that has IIS.
Cassini (developers web server) has limitations and doesn't serve http requests from intranet/internet
You should deploy your site to IIS on your workstation (if your friends observe workstation via IP) or use Internet Asp.net Hosting
The development server can only be accessed by clients on the local box. You'll need to host your site on IIS to let them access it (assuming they're on your local network.) If you need to expose this on the internet, you'll have to get a hosting company to do that.
You are probably using the built-in webserver called cassini. By default, that webserver does only accept request from the localhost.
Set up a local IIS, and forward all necessary ports through the firewall
The development server will only server connections from localhost. You need to deploy it to a real IIS server instance. Since you don't likely have Windows Server lying around, you can install IIS on any Professional edition of windows. Then your site will run on port 80, just like any other web site. You still may also need to forward that port on your home router.