Sigh. This is going to be a long post. For whatever it’s worth - I’m a Cisco Certified Internetwork Engineer, with approx. 25 years networking experience. Not bragging - just opening w/ that so we all understand this is not a “turn it off and on again” kind of scenarios 
I have a mostly Cisco network at home - switches, firewall, routers, etc. Connected to this network, a bunch of devices - some wired, some wireless. Some use static IP addresses (like NAS boxes). Some (clients, Apple TV, Xbox) use DHCP. Nest cameras, thermostats and protects use DHCP. The RACHIO uses DHCP too.
My switch running Cisco IOS works as a DHCP server. It has a set of excluded addresses - from .1 to .127 are excluded. It only assigns/offers addresses from .128 to .254. So far, so good.
The Rachio gen 2 controller, configured for DHCP, is for some unknown reason insisting to use .100 - which is *already assigned, statically, to a NAS box *. So today I can’t access the NAS - only to find out, again, the Rachio is using the same address - WHICH DID NOT GET FROM THE DHCP SERVER.
Took me a while to track down it was the Rachio box - had to start looking across a truckload of devices for their MAC addresses, 'til I finally got to the Rachio. Surprise ! There’s no way, no way to know which IP address the Rachio box is using ! But I could see the ARP binding between 192.168.1.110 and the RACHIO MAC address.
And here’s the kicker - after about 30 mins of troubleshooting, the Rachio stopped using the 192.168.1.110 IP Address - now I can access the NAS again. Why did it stop using it? MISTERY.
So I did a ping to 192.168.1.255 - and thanks God for small miracles, the Rachio IP stack does reply to a ping to the network broadcast address. So I know it is now using 192.168.1.220 . . . . but why ?
Is there any way to troubleshoot this further ?
