Connect your iPhone to your Mac via USB.
Go to Network Utility (cmd+space and type "network utility")
Go to the "Info" tab
Click on the drop down menu that says "Wi-Fi" and select "iPhone USB" as shown here:
You'll find an IP address like "xxx.xxx.xx.xx" or similar. Open Safari browser on your iPhone and enter IP_address:port_number
Example: 169.254.72.86:3000
[NOTE: If the IP address field is blank, make sure your iPhone is connected via USB, quit Network Utility, open it again and check for the IP address.]