If an interface is configured with a manual or static IP address, you can also add secondary static IP addresses to the interface. Adding secondary IP addresses effectively adds multiple IP addresses to the interface. Secondary IP addresses cannot be assigned using DCHP or PPPoE.
All of the IP addresses added to an interface are associated with the single MAC address of the physical interface and all secondary IP addresses are in the same VDOM as the interface that are added to. You configure interface status detection for gateway load balancing separately for each secondary IP addresses. As with all other interface IP addresses, secondary IP addresses cannot be on the same subnet as any other primary or secondary IP address assigned to a FortiGate interface unless they are in separate VDOMs.
To configure a secondary IP, go to System > Network > Interface, select Edit or Create New and select the Secondary IP Address check box.