0 means that ur server connects to external ip so u can connect from other sources
to demonstrate if you run the server host on 8000 without the 0 and do ‘curl localhost:8000’ on same server running the app u will see the content, but if you run ‘curl <server_ip>:8000’ from another device u will get forbidden because u r only allowing internal devices to connect when you don’t specify the 0, 0 means
0.0.0.0