On an IIS server, with a simple static web page, I constantly got: HTTP Error 503. The service is unavailable I did not get any entry in the IIS logs and when stopping the respective app pool, I got the same message. There was definitely no port conflict as I checked with TCPView if any other process was using port 80. After a while I tried to change the binding to port 81. And - bingo - , on port 81, the web site worked. After digging for a while, I found that port 80 had an "ACL reservation". Read more [...]