Edited sentence: (The name Logical Unit Number is historical; it is not a number).A large storage system may have up to thousands of logical units, defined logically, by administrative command, and the administrator may choose the LUN or the system may choose it.And labels or serial numbers stored in a logical unit's storage volume often serve to identify the logical unit.But LUN zero need not provide any other services, such as a storage volume.However, the LUN is the only way for an initiator to address a command to a particular logical unit, so initiators often create, via a discovery process, a mapping table of LUN to other identifiers.In current SCSI, a LUN is a 64 bit identifier.In current SCSI, the initiator delivers the CDB to a particular  logical unit, so the LUN appears in the transport layer data structures and not in the CDB.In earlier versions of SCSI, and with some transport protocols, LUNs are 16 or 6 bits.In the earliest versions of SCSI, an initiator delivers a Command Data Block (CDB) to a target (physical unit) and within the CDB is a 6 bit LUN field to identify the logical unit within the target.It is divided into 4 16 bit pieces that reflect a multilevel addressing scheme, and it is unusual to see any but the first of these used.Many SCSI targets contain only one logical unit (so its LUN is necessarily zero).Others have a small number of logical units that correspond to separate physical devices and have fixed LUNs.People usually represent a 16 bit single level LUN as a decimal number.So two computers that access the same disk volume may know it by different LUNs.The LUN identifies a logical unit only within the context of a particular initiator.The LUN is not the only way to identify a logical unit.The logical unit with LUN zero is special in that it must implement a few specific commands, most notably Report LUNs, which is how an initiator can find out all the other LUNs in the target.There is also the SCSI Device ID identifies a logical unit uniquely in the world.There is one LUN which is required to exist in every target: zero