I have installed Apache server on my Windows 7 computer. I was able to display the default index.php by typing http://localhost/ in the address line of my browser.

However, I am still unable to see this page by typing IP address of my computer (neither locally (from the same computer) no globally (from another computer connected to the Internet)).

I was told that I need to open port 80. I did it (in a way described here) but it did not solve the problem.

First of all I would like to check which ports are opened and which are not. For example I am not sure that my port 80 was closed before I tried to open. I am also not sure that it is opened after I tried to open it.

I tried to run a very simple web server written in Python. For that I used port 81 and it worked! And I did not try to open the port 81. So, it was opened by default. So, if 81 is opened by default, why 80 is not? Or it is?

ADDITIONAL INFORMATION:
1. In my httpd.conf file I have "Listen 80".
2. This site tells me that port 80 on my computer is opened.
3. I get different responses if I try http://myip:80 and http://myip:81. In the last case browser (Chrome) writes me that link is broken. In the first case I get: Forbidden You don't have permission to access / on this server.
4. IE writes that "The website declined to show this webpage".