Customer Support

There are more then just those 2 reasons. They might not have a
default/index page specified, or the server may be looking for
index.html and they have default.html or something similar.

Another way to get 403 Forbidden: inadvertently reset your Apache
options.

If you are running the CGI version of php, you need the Apache directive
Options ExecCGI
which modwest sets for you.

But now if you need some other option and put a new Options directive in
one of your htaccess files, be sure you include a + to indicate you're
just adding a new option.  Without the + you're resetting all your
options to just the ones in your current list.  

In my case I installed Gallery (an excellent php package) and Gallery
said 
Options FollowSymLinks
and this undid the ExecCGI option and caused my pages to be forbidden. 
The solution in my case is
Options +FollowSymLinks.

Also, if you try to use a .htaccess override on an Apache server that
has these overrides disabled you will get a 403, Forbidden error

You may need to include the allowed IP addresses and not just the port.
If on an internet router network with NAT or whatever, try changing the
port to something other than 80 so that the router config doesn't pick
it up.
once you've done that, modify the apache config file to allow your other
computers/network ip addy's.
this can be in any ip range format as defined in the following link
http://httpd.apache.org/docs/2.0/mod/mod_access.html