World Wide Name

A World Wide Name (WWN) or World Wide Identifier (WWID) is a unique identifier used in storage technologies including Fibre Channel, Advanced Technology Attachment (ATA) or Serial Attached SCSI (SAS).

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. Two WWNs which do not refer to the same thing should always be different even if the two are used in different roles, i.e. a role such as WWPN or WWNN does not define a separate WWN space. The use of burned-in addresses and specification compliance by vendors is relied upon to enforce uniqueness.

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 (often the term "Company Identifier" is used as a synonym for OUI) and vendor-supplied information. Each format defines a different way to arrange and/or interpret these components. OUIs are used with the U/L and multicast bits zeroed, or sometimes even omitted (and assumed zero.)

The WWN formats include:

WWN addresses are predominantly represented as colon separated hexadecimal octets, MSB-first, with leading zeros — similar to Ethernet's MAC address. However, there is much variance between vendors.

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. That is an advantage when the WWNs are used in configuration files and scripts, e.g. /etc/fstab.

OUIs can be queried searching the IEEE organization's Public Manufacturers OUI list.

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