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The Ethernet MAC address FF:FF:FF:FF:FF:FF is reserved for broadcasts. If all frames are naturally broadcast in a LAN, what is the need for a broadcast address?

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  • $\begingroup$ frames are not naturally broadcast. $\endgroup$ – yoyo_fun Feb 13 '17 at 14:37
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    $\begingroup$ you may also wantto ask this question on network engineering stack exchange site. you may receive more complete answers there $\endgroup$ – yoyo_fun Feb 13 '17 at 14:42
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So that receiving devices can blindly throw away packets that aren't addressed to them or the broadcast address. Doing this test is much simpler than having to verify the CRC and tossing it up to the next layer that may toss it out itself.

A switch will also make use of the target address to only propagate packets to where it knows the destination address resides, reducing congestion.

The broadcast destination is used for stuff like DCHP where the sender doesn't know the target's MAC but still needs to send out the message.

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The purpose is to inform the recipients that they should receive rather than ignore the message.

While Ethernet messages are "naturally broadcast", this is an implementation detail. Most messages are intended only for one recipient, and just happen to be receivable by the other addresses connected to the LAN.

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  • $\begingroup$ what does naturally broadcast mean ? $\endgroup$ – yoyo_fun Feb 13 '17 at 14:40
  • $\begingroup$ @yoyo_fun It's a quote of the original question. And it refers to the fact that that the Ethernet hardware broadcasts all frames to all attached stations. $\endgroup$ – Martin Berger Feb 14 '17 at 9:22
  • $\begingroup$ Correct me if I am wrong but the only ethernet hardware that broadcasts the frames to every attached device is the an ethernet hub which is rarely used now. So the statement "Ethernet messages are naturally broadcast" is factually wrong. $\endgroup$ – yoyo_fun Feb 14 '17 at 10:14
  • $\begingroup$ @yoyo_fun Correct me if I am wrong but, ass far as I'm aware, Ethernet's underlying network technology has not changed recently and is still based on broadcasting. What has changed is how people use Ethernet. As you correctly point out, today 'star-topologies' are popular, where you have a switch is connected to each device via a separate Ethernet LAN. The purpose of this setup is to minimise the probability of message collisions (two stations sending at the same time). But that's just a choice, the two stations on each LAN (switch and desktop, say) still broadcast. $\endgroup$ – Martin Berger Feb 14 '17 at 18:21
  • $\begingroup$ How is sending the data from the NIC to outside network called broadcasting ??? What would be an example of non broadcasting communication ? I do not understand the difference between broadcasting and non broadcasting. Why is sending the packet to the Switch called broadcasting when it is simply just sending the package out t be switched and routed to the destination. $\endgroup$ – yoyo_fun Feb 14 '17 at 18:52

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