The port to listen on. Set to `0` to listen on any available port.

A literal IP address or host name that can be resolved to an IP address. __Note about `0.0.0.0`__ While listening `0.0.0.0` works on all platforms, the browsers on Windows don't work with the address `0.0.0.0`. You should show the message like `server running on localhost:8080` instead of `server running on 0.0.0.0:8080` if your program supports Windows.