Explanation below:
PC (host) = 192.168.178.20 (static IP),
Freetz_linux = 192.168.178.21 (IP from dhcp of fritz.box)
Freetz.box = 192.168.178.1 (static, dhcpd sever)
Unfortunately, I am not able to make the static IP for Freetz_linux, even if I update the /etc/network/interfaces with:
auto...