@giminik You can assign subdomains at the interface's DHCP server tab.

Domain name: "The default is to use the domain name of this system as the default domain name provided by DHCP. An alternate domain name may be specified here."

So you can end up with
-firewall.home or firewall.lan.home
-firewall.lan2.home
-firewall.dmz1.home
-firewall.dmz2.home

@Rico
word! i do not need to unserstand why i would do this ;)
CSO local networks but here in ausrtia a lot of things are possible ;)