Thus, by definition, an EUI is not in fact a "MAC address", although it is syntactically indistinguishable from one and assigned from the same numbering space. The IEEE now considers the label MAC to be an obsolete term, previously used to refer to a specific type of EUI identifier used to address hardware interfaces within existing based networking applications, and thus not to be used in the future. Instead, the proprietary term EUI should be used for this purpose. The IEEE has built in several special address types to allow more than one network interface card to be addressed at one time:.
These are all examples of group addresses , as opposed to individual addresses ; the least significant bit of the first octet of a MAC address distinguishes individual addresses from group addresses. That bit is set to 0 in individual addresses and set to 1 in group addresses. Group addresses, like individual addresses, can be universally administered or locally administered.
In both cases, the process can be trivially reversed when necessary. Organizations issuing EUIs are cautioned against issuing identifiers that could be confused with these forms. Although intended to be a permanent and globally unique identification, it is possible to change the MAC address on most modern hardware. Changing MAC addresses is necessary in network virtualization. It can also be used in the process of exploiting security vulnerabilities. This is called MAC spoofing.
A host cannot determine from the MAC address of another host whether that host is on the same link network segment as the sending host, or on a network segment bridged to that network segment. On broadcast networks, such as Ethernet, the MAC address uniquely identifies each node on that segment and allows frames to be marked for specific hosts. It thus forms the basis of most of the link layer OSI Layer 2 networking upon which upper layer protocols rely to produce complex, functioning networks.
Layer 2 switches use MAC addresses to restrict packet transmission to the intended recipient. However, the effect is not immediate. However, since IEEE For example, an address in canonical form A-BC would be transmitted over the wire as bits in the standard transmission order least significant bit first. But for Token Ring networks, it would be transmitted as bits in most-significant-bit first order. The latter might be incorrectly displayed as C-6A-1ED.
Canonical form is generally preferred, and used by all modern implementations.
When the first switches supporting both Token Ring and Ethernet came out, some did not distinguish between canonical form and non-canonical form and so did not reverse MAC address bits as required. This led to cases of duplicate MAC addresses in the field. Information is freedom. Freedom is non-negotiable.
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