Why destination mac address is first in ethernet frame?
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The reason why the destination MAC is the first element of an Ethernet frame is to allow Ethernet transceivers to be able to tell quickly whether or not any given frame is "for it".
Ethernet is a broadcast medium: every device on a segment receives every frame transmitted on that segment, except for ones that it itself transmits, whether or not that frame is intended for it.
Putting the destination MAC first allows each transceiver to determine by listening to the first six bytes of the frame whether it needs to continue listening to the frame to actually receive it for processing by the device the interface is attached to, by comparing the destination address with its own MAC; if the destination address is either its own MAC or is the all-ones Ethernet broadcast address, the interface needs to continue listening and actually receive the frame; otherwise, it does not and can discard the frame.
Ethernet is a broadcast medium: every device on a segment receives every frame transmitted on that segment, except for ones that it itself transmits, whether or not that frame is intended for it.
Putting the destination MAC first allows each transceiver to determine by listening to the first six bytes of the frame whether it needs to continue listening to the frame to actually receive it for processing by the device the interface is attached to, by comparing the destination address with its own MAC; if the destination address is either its own MAC or is the all-ones Ethernet broadcast address, the interface needs to continue listening and actually receive the frame; otherwise, it does not and can discard the frame.
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