A World Wide Name (WWN) or World Wide Identifier (WWID) is a unique identifier used in storage technologies including Fibre Channel, Parallel ATA, Serial ATA, SCSI and Serial Attached SCSI (SAS).
[1] A WWN may be employed in a variety of roles, such as a serial number or for addressability; for example, in Fibre Channel networks, a WWN may be used as a WWNN (World Wide Node Name) to identify an endpoint, or a WWPN (World Wide Port Name) to identify an individual port on a switch.
Each WWN is an 8- or 16-byte number, the length and format of which is determined by the most significant four bits, which are referred to as an NAA (Network Address Authority).
The remainder of the value is derived from an IEEE OUI (or from Company Id (CID)) and vendor-supplied information.
[4] Linux uses WWN to identify disks by providing symbolic links to the real device entry: (There are more entries in this directory which are omitted here) The target names (sr0, sda) might change when new devices are added to the computer (e.g. sda might become sdb) but the WWN will be the same.