actually your problem lays in the apache not the router or firewall. if you are wanting to, for example, locally make and test a website you have to tell the apache to listen on that ip address. to access it by "localhost" and get the xampp homepage works because the httpd.conf is already listening on that address (127.0.0.1).
add: Listen 192.168.1.100:80
enable: virtual hosting
enable: name virtual host
set up and point to the host.
you can name the site whatever you want as long as the system knows where to go.
hope this helped