Good article, but the easiest way to get the IP address of your local server is to punch in:

ipconfig getifaddr en0

To your Terminal (mac). It returns a number like 192.168.8.103, and then as you say in your article, you add on your port number such as :8080 or :3000. Then put it into your mobile browser search bar, as in:

192.168.8.103:3000

Easy peasy!