At the beginning of this let me preface that I am a networking noob. The last time that I setup a router was 6-7 years ago with my still working USR8200 which has required no fiddling ...
OK, so I have a subnet (192.168.11.0) that currently has all servers and clients on it. It's running out of addresses. So, I create a new subnet (192.168.12.0) and change my DHCP server (Fortigate ...