Ok we really need to tackle this @spetrosi @mcorr and @ekohl. IMHO users need to be told in the planning guide and/or installation guide how to handle this.
Stefan, unfortunately Foreman does not perform any subnet management, you need to take care of this. Our installer only supports one subnet, then you need to use custom-hiera.yaml to add multiple subnets. Foreman only uses OMAPI to talk with DHCP server creating records within those subnets.
The above statement should be probably in the Planning Guide. Then we need to put this into the Installation Guide:
https://theforeman.org/2017/07/adding-new-subnet-for-provisioning.html
@ekohl to confirm if these instructions are still valid.