This article outlines how to rectify the issue "Please enter a valid IP" when registering a network on the Umbrella dashboard.
The error typically pops up when the IP address range that is being registered has an invalid CIDR notation:
You will have to confirm the valid CIDR notation for your IP address range. You may use any 3rd party website to verify the CIDR notation that you are using is correct. An example is this 3rd party website that will provide you with the correct CIDR notation.