Origin200 = NIC the MSC (system controller board, the part with the buttons on it under the 5.25" bays)
Octane = NIC on the frontplane board.
These NICs aren't network boards, they're small SEPROMs that look like lithium batteries. They can be removed and swapped if you need to replace the part they're on or want to change your serial number for another reason.
Indigo2 8-pin DIP ROM on the serial/keyboard port riser board.
Indigo: on the backplane
Personal IRIS: 8-pin DIP PROM on the board with the power and fault LEDs.
Indy: stored in the Dallas chip in a similar fashion to the Sun hostid. It is equal to the Ethernet MAC address listed on the back yellow/orange label if you need to reset it.
Onyx/CHALLENGE: The hostid/serial number is stored on the system controller in a Dallas chip. Can be reprogrammed using the PROM monitor