You can have processes listen on 0 - which means all available IPv4 IPs (available to that machine). And since you can have processes listen on 0, it means you

You can have processes listen on 0 - which means all available IPv4 IPs (available to that machine). And since you can have processes listen on 0, it means you can equally curl 0; just like you could with 127.0.0.1. Here's an example from my IRC server (the only process I run on 0): $ curl https://0:9997 -kis | head HTTP/1.1 200 OK Date: Fri, 12 Feb 2016 07:49:24 GMT Server: ZNC - http://znc.in Content-Length: 1878 Content-Type: text/html Set-Cookie: 9997-SessionId=54245f15ba592bc691e09ac75e6778e6d4c33841fad71a8d6c56addc998e043f; path=/; Connection: Close <?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> (just in case someone queries, 0 is just another notation of 0.0.0.0)