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Why do HBAs in a SAN have same base?

Storage expert Chris Poelker outlines WWN basics in order to answer the question: "Why do HBAs in a SAN have same base?"

Why do HBAs in a SAN have same base?
In a SAN, a world wide name (WWN) is a 64-bit identifier for each device in the fabric.

All WWNs are registered with and assigned by the The Institute of Electrical and Electronics Engineers (IEEE). This is done to assure each and every WWN is unique. A WWN is similar in concept to a network card's MAC address in an IP network, but is formatted differently.

WWNs are formatted as follows:

WWN= XYYYYYYZ ZZZZZZZZ (Example: 500060E8064183F3)

X = the first 4-bit field, called the NNA, specifies the format the WWN follows. This number is a "5" for storage target ports, and either a "1" or a "2" for an HBA port.

YYYYYY = called the organizational unique identifier (OUI) which is assigned by the IEEE to specific vendors (Such as Qlogic, Emulex, Hitachi, EMC, IBM, etc...) This is why the address in this area always seems to be similar if you are buying from a specific vendor.

ZZZZZZZZZ = the vendor specified identifier (VSI), which is defined by the vendor owning the OUI. These numbers can be changed by the vendor to develop an offset of the node WWN to get a port WWN. As an example, an HBA may have dual ports. The HBA is assigned a node WWN, and an offset is used to get the port WWN for each port on the HBA by changing the value of the VSID.

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