The instructions below detail the process for creating a host reservation via the NS1 Connect portal. For programmatic instruction, refer to this article.
Create or select an existing subnet (status=assigned) in the network (e.g. 10.0.0.0/28).
Under that subnet, create a child subnet (status=assigned) with a prefix length contained within the above-assigned address (e.g. 10.0.0.1/32).
The subnet prefix length must be /32 in order to be added as a reservation.
From the list of subnets, navigate to the child subnet you just created.
Click the menu (three-dots) icon, and select Create host reservation.
In the Create host reservation dialog box, choose an existing scope group from the drop-down menu and enter the MAC address for the associated host.