A LUN is a unique identifier given to separate devices, or logical units, so they can be accessed by a SCSI, iSCSI or Fibre Channel protocol. A logical unit number (LUN) is a unique identifier when combined with information such as the target port identifier for designating an individual or collection of physical or virtual storage devices that execute input/output (I/O) commands with a host computer, as defined by the Small System Computer Interface (SCSI) standard.