Why are you making such a negative comment? I am a student intern my first day and I choose the highest plan in netcup and paid in full. (This money I earn by working in the pub) ...if you don't help me, don't say bitter words. it can hurt others, just as others hurt you with words...
I don't see how your source of money has anything to do with this.
And I don't see any harsh comments.
You are renting a server with a direct 2.5 GBit/s connection to the internet. Handling this, protecting the server against attacks, requires a lot of responsibility and knowledge.
Usually one would train how to use Linux in a protected environment, like a virtual machine on your own computer, or something like a Raspberry Pi.
Your stuggle with setting up network (without reading the documentation provided by netcup and CentOS) may lead to the impression, that this product might not be right for you. After all you are requesting our help here, which we all do in our time off.
This has been communicated very respectfully and in a kind manner. We don't want to disrespect you.
Maybe you would understand more by reading this: http://www.catb.org/~esr/faqs/smart-questions.html
If you don't want to accept that, unfortunately you are on your own - at least in this board.
Good luck.