That is correct, you’ll need to enter the 184.108.40.206 as the subnet.
You are aware that the subnet you selected for your private IP are public routable IPs. By using those IPs for your private network you won’t be able to access sites which are hosted on those IPs on the public Internet. Those public IPs are owned by Agilent Technologies and Hewlett-Packard.
Like Denny pointed out – those IPs are severely Public. If you need to use the /20 subnet mask, then might I suggest using the 10.x.x.x range? you could alter your numbering schem to something like 10.25.48.0 /20 (255.255.240.0)…