why do we assign secondary IP address

Hi @Umesh, 

If you would like to have multiple IP address on the interface, you can make use of secondary IP addresses. This helps in configuring multiple subnets on the same interface. 

https://docs.fortinet.com/document/fortigate/6.0.0/handbook/915482/secondary-ip-addresses-to-an-interface

Thank you. 

Shahan

Usually it is a temporary state, when for example switching topology , say from LAN 10.10.10.0/24 to 192.168.15.0/24 and you want to minimize downtime. Configuring new network on the FGT interface to LAN you can switch lans and both networks will work without changing something in the fortigate. 

another use case could be the following: you need access to the internet, but the p2p subnet between the FW and the router needs to be private, so that it can't be attacked from the internet. Then you configure a public secondary ip, and use it for VPNs or sdwan.