In a 1+1 High Availability (HA) deployment, FortiNAC-F supports two primary methods for management access: individual IP addresses or a Shared IP Address (also known as a Virtual IP or VIP). The Shared IP option is part of a Layer 2 HA design, which simplifies administration by providing a single URL or IP that always points to whichever appliance is currently in the "Active" or "In Control" state.
For a Shared IP configuration to function correctly, the Primary and Secondary administrative interfaces (port1) must be on the same subnet. This requirement exists because the Shared IP is a logical address that is dynamically assigned to the physical interface of the active unit. Since only one unit can own the IP at a time, both units must reside on the same broadcast domain (Layer 2) to ensure that ARP requests for the Shared IP are correctly answered and that the gateway remains reachable regardless of which unit is active. If the appliances were on different subnets (a Layer 3 HA design), a shared IP could not be used because it cannot "float" across different network segments; instead, administrators would need to manage each unit via its unique physical IP or use a FortiNAC Manager.
"For L2 HA configurations, click the Use Shared IP Address checkbox and enter the Shared IP Address information... If your Primary and Secondary Servers are not in the same subnet, do not use a shared IP address. The shared IP address moves between appliances during a failover and recovery and requires both units to reside on the same network." - FortiNAC-F High Availability Reference Manual: Shared IP Configuration.