If port 80 is already in use
How to Change HTTP Listening Port in Apache
Last week I came across a server where HTTPS port 80 was already taken by another software. The "culprit" was, not surprisingly, another web server software, namely IIS. Since the IIS users are already used to the standard port, the Apache installation had no choice but to use a less popular port for HTTP, in this case port 81.
You may be wondering why port 80 is important for http? Well, first of all, it is for the convenience of the users. If you are surfing to a web server that is listening on a port other than port 80, you will need to specify the port number along with the address.
Something like this: http://yourdomain.com:81/ if port 81 is your web server's port. If you are using port 80, you do not need to type port 80 into the address because that is the standard port used for HTTP. According to http://www.tcpipguide.com
