Denic (.DE Registry) has certain requirements for nameservers that can be applied to .DE domains. A minimum of 2 authoritative independent DNS is required. It means that the IP-addresses of each nameserver should differ by at least one number in any octet. Also, at least one of them must be connected via IPv4.
192.0.2.1 and 192.0.2.2 pair is valid (IPs differ in 1 digit here);
192.0.2.1 and 2001:db8:85a3::8a2e:370:7334 pair is also valid.
Denic has created the NAST tool where you can check if your nameservers are compatible with a .DE domain.You need to enter the domain name in question and its nameservers into the corresponding fields. If no error is reported, you can then set these nameservers for your domain using this guide.
If you have any questions, feel free to contact our Support Team.