It all depends how you've configured apache. The default index.html for apache is the "it works!" page.
It sounds like you've assigned the external IP address of your server to one of the sites you're hosting (eertwegh.nl). This has made that site the default site to be displayed if anyone navigates to the IP address of your server or uses a domain name that has a DNS record pointing to the server with no site configured as is your setup below.
I'd check the default config file in /etc/apache2/sites-available. Make sure that it shows <Virtualhost *:80>. Then check the eertwegh.nl vhost file in the same directory. Make sure it shows <Virtualhost *:80> as it sounds like it's probably showing <Virtualhost 126.96.36.199:80> (where 188.8.131.52 is your external ip address).
Assuming that all host files show *:80 then the default index page will be displayed when navigating to the IP address of the server. You might want to change the default page to something more descriptive or personalised than the "it works!" page but that's just my personal choice.
Any problems or questions then let me know.