An Extended Unique Identifier (EUI) is either a 48-bit Extended Unique Identifier (EUI-48) or a 64-bit Extended Unique Identifier (EUI-64). With some exceptions, particularly with regard to protocol identifiers, each EUI is intended to be globally unique and bound to a hardware device instance or other object that requires unique identification. EUI-48 and EUI-64 identifiers are most commonly used as globally unique network addresses (sometimes called MAC addresses), as specified in various standards. For example, an EUI- 48 is commonly used as the address of a hardware interface according to IEEE Std 802, historically using the name “MAC-48”. As another example, an EUI- 64 may serve as the identifier of a clock, per IEEE Std 1588. IEEE Std 802 also specifies EUI-64 use for 64-bit globally unique network addresses. Further detail regarding EUI-48 and EUI-64 is provided below. When an EUI is used as a MAC address (for example, an IEEE 802 network address), the two least significant bits of the initial octet (Octet 0) are used for special purposes. The least significant bit of Octet 0 (the I/G bit) indicates either an individual address (I/G=0) or group address (I/G=1), and the second least significant bit of Octet 0 (the U/L bit) indicates universal (U/L=0) or local (U/L=1) administration of the address. A universally administered address is intended to be a globally unique address. In an EUI created by extending an OUI, the OUI is the initial (most significant) three octets. In an EUI created by extending an OUI-36, the OUI-36 is the initial (most significant) four and a half octets. Since OUI and OUI-36 assignments made by the IEEE RA have the X bit equal to 0, an EUI created as an extended identifier from an assigned OUI or OUI-36 has U/L=0 and, when used as a MAC address, is thus a universally administered address. Since all OUI and OUI-36 assignments made by the IEEE RA have the M bit equal to 0, an EUI created as an extended identifier from an assigned OUI or OUI-36 has I/G=0 and, when used as a MAC address, is thus an individual address. The assignee of an OUI or OUI-36 is exclusively authorized to assign group MAC addresses, with I/G=1, by extending a modified version of the assigned OUI or OUI-36 in which the M bit is set to 1. Such addresses are not EUIs and do not globally identify hardware instances, even though U/L=0.
An Extended Unique Identifier (EUI) is either a 48-bit Extended Unique Identifier (EUI-48) or a 64-bit Extended Unique Identifier (EUI-64). With some exceptions, particularly with regard to protocol identifiers, each EUI is intended to be globally unique and bound to a hardware device instance or <br>other object that requires unique identification. EUI-48 and EUI-64 identifiers are most commonly used as globally unique network addresses (sometimes called MAC addresses), as specified in various standards. For example, an EUI- 48 is commonly used as the address of a hardware interface according to IEEE Std 802, historically using the name “MAC-48”. As another example, an EUI- 64 may serve as the identifier of a clock, per IEEE Std 1588. IEEE Std 802 also specifies EUI-64 use for 64-bit globally unique network addresses. Further detail regarding EUI-48 and EUI-64 is provided below. <br>When an EUI is used as a MAC address (for example, an IEEE 802 network address), the two least significant bits of the initial octet (Octet 0) are used for special purposes. The least significant bit of Octet 0 (the I/G bit) indicates either an individual address (I/G=0) or group address (I/G=1), and the second least significant bit of Octet 0 (the U/L bit) indicates universal (U/L=0) <br>or local (U/L=1) administration of the address. A universally administered address is intended to be a globally unique address. <br>In an EUI created by extending an OUI, the OUI is the initial (most significant) three octets. In an EUI created by extending an OUI-36, the OUI-36 is the initial (most significant) four and a half octets. Since OUI and OUI-36 assignments made by the IEEE RA have the X bit equal to 0, an EUI created as an extended identifier from an assigned OUI or OUI-36 has U/L=0 and, when used as a MAC address, is thus a universally administered address. Since all OUI and OUI-36 assignments made by the IEEE RA have the M bit equal to 0, an EUI created as an extended identifier from an assigned OUI or OUI-36 has I/G=0 and, when used as a MAC address, is thus an individual address. The assignee of an OUI or OUI-36 is exclusively authorized to assign group MAC addresses, with I/G=1, by extending a modified version of the assigned OUI or OUI-36 in which the M bit is set to 1. Such addresses are not EUIs and do not globally identify hardware instances, even though U/L=0.
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