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In the latest SCSI transport protocols, there is an automated method of "discovery" of the IDs. SSA initiators "walk the loop" to determine what devices are there and then allocate each one a 7-bit "hop-count" value. FC-AL initiators use the LIP to interrogate each device port for its WWN. For iSCSI, because of the unlimited scope of the network, the technique is quite complex.

On a parallel SCSI bus, a device is recognized 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 former 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 holds a BIOS program that runs when the computer boots up and that program has menus that let the operator decide on the SCSI ID of the host adapter.

The SCSI ID of a device in a drive enclosure that has a backplane is set either by jumpers or by the slot in the enclosure the device is installed into, relying 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 pick a unique SCSI ID.

Note that a SCSI target device is often divided into smaller "logical units." For example, a high-end disk subsystem may be a only SCSI device but contain dozens of private disk drives, each of which is a logical unit.The SCSI ID, WWN, etc. in this case identifies the entire subsystem, and a second number, the logical unit number identifies a disk device within the subsystem.

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Finally setting the bootable hard disk to nice SCSI ID 0 is an accepted IT community recommendation. ID 2 of SCSI is usually set aside for the floppy disk drive.

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