Assign Interface

If you only have 1 ethernet card, this should automatically be picked up. No need to do anything at all.

If you have more than 1 interface:

From the initial Console Setup menu, press 1 to enter the LAN setup.

Select the interface you want to use to connect to the network.

Then select exit.

Time to set your FreeNAS IP now.

Ref: FreeNAS wiki