In the current SCSI transport protocols, there is an automated method of “discovery” of the IDs. SSA initiators “walk the loop” to settle on what devices are there and then allocate each one a 7-bit “hop-count” value. FC-AL initiators exercise the LIP to interrogate each device port for its WWN. For iSCSI, because of the infinite scope of the network, the process is quite difficult.
On a parallel SCSI bus, a device is identified by a “SCSI ID”, which is a number in the range 0-7 on a narrow bus and in the range 0-15 on a wide bus. On earlier models a physical jumper or switch controls the SCSI ID of the initiator.
On modern host adapters, doing I/O to the adapter sets the SCSI ID; for example, the adapter often contains a BIOS program that runs when the computer boots up and that program has menus that let the operator select the SCSI ID of the host adapter.
The SCSI ID of a device in a drive enclosure that has a backplane is set both by jumpers or by the slot in the enclosure the device is installed into, depending on the model of the enclosure. In the latter case, each slot on the enclosure’s back plane delivers control signals to the drive to select a unique SCSI ID.
Note that a SCSI target device is often seperated into smaller “logical units.” As example, a high-end disk subsystem may be a single SCSI device but include dozens of private disk drives, each of which is a logical unit.The SCSI ID, WWN, etc. in this case identifies the whole subsystem, and a second number, the logical unit number identifies a disk device within the subsystem.
Finally setting the bootable hard drive to nice SCSI ID 0 is an accepted IT community recommendation. SCSI ID 2 is usually set aside for the floppy disk drive.