Ethernet: Address Formats

In order for a MAC address to be unique the IEEE assigns each vender a code to use as the first 3 bytes of the MAC.  This is called the Organizationally Unique Identifier (OUI).  Along with the OUI the vender then assigns a unique value in the low-order 3 bytes for each Ethernet card  that it manufactures, thereby ensuring global uniqueness of MAC addresses.

OUI

IEEE documentation lists the Ethernet addresses with the most significant byte on the left.  Inside each byte, the leftmost bit is the most significant bit and the rightmost bit is the least significant bit.  This is referred to as the canonical bit order.

I/G bit – binary 0 means that the address is a unicast, binary 1 means that the address is Multicast or Broadcast

U/L bit – Binary 0 means that the address is vender assigned, Binary 1 means that the address has been administratively assigned, overriding the vender assigned address.

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