Configuring the management interfaces
The default FortiGate-6000 configuration includes a management VDOM named mgmt-vdom. The management interfaces mgmt1, mgmt2, and mgmt3 are all in this VDOM. The ha1 and ha2 interfaces are also in this VDOM. This mgmt-vdom cannot be removed or renamed and interfaces cannot be removed from it or added to it. Fortinet recommends managing the FortiGate-6000 from the mgmt1, mgmt2, and mgmt3 out-of-band management interfaces and not from a data interface.
You can change the IP address of the management interfaces and configure routing for the management VDOM as required. For example, from the GUI, select the mgmt-vdom then go to Network > Interfaces to configure the mgmt1, mgmt2, or mgmt3 interfaces.
From the CLI:
config system interface
set ip <ip-address> <netmask>