How can I make it that my local Apache installation works only as http://localhost/ and never from outside like using the IP of my machine when connected to a network please?
Change your current "Listen" line to "Listen 127.0.0.1:80"
Some of this is borrowed from httpd.apache.org/docs/2.2/misc/security_tips.htm.
Add the following to httpd.conf :
Restrict access to everything by default. This is from "Protect Server Files by Default":
<Directory /> Order Deny,Allow Deny from all </Directory>
Then, allow access only in those areas you wish. In this example,
/var/www/htmlis my DocumentRoot:
DocumentRoot "/var/www/html" <Directory /var/www/html/> Order Deny,Allow Allow from 127.0.0.1 </Directory>