Welcome to LQ! You need to provide us more information on what is it exactly you are trying to do. What you have asked is to explain concepts instead of giving tried and tested scenarios and any error messages you received while trying.
To answer you questions
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a) I read that its possible to use one network card with 2 IP ranges. Can you explain how that works? In my opinion I am only able to adress one IP adress to a specific adapter.
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On a basic level a firewall is used to protect a network which requires 2 network cards. 1 for the internal network (your LAN) and another for the external network (The internet)
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I read a concept with outer and inner firewall on a software basis. Can you explain that for me, please?
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A software firewall is basically a computer or similar with 2 network cards that has been configured to perform this function. You can buy dedicated firewall devices that would be classfied a a hardware firewall however even those do run some sort of software as a Fortinet or similar.
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If I have two network cards and I want to route from one adapter to another, how do I configure that in Iptables? Don't I have to create a route outside iptables?
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Most firewall documentation will guide you on how to do this
Keep practicing and ALWAYS read the manuals