What do you do if you want to ask everyone a question? You shout! Thats pretty much a broadcast. Why would you want to shout? Maybe you know someones name, but not what their face looks like, so you shout their name and hopefully that person will turn around and address you directly with their reply (unicast). Others will hear your cry, be annoyed, but probably won't reply. This is a simple analogy of ARP on an Ethernet network (you know the IP address, but not the MAC address). A layer 2 broadcast is normally a reflection of the Layer 3 broadcast - if I ping 172.16.255.255, this will normally be sent on the wire as MAC address ffff.ffff.ffff. Some applications require/are written for broadcast. It depends what the application wants to achieve (RIPv1 is the first to spring to mind).
Not sure for the semantics of your last question, but generally a layer 2 broadcast would be on a LAN. A layer 3 broadcast is classified as the host portion of the IP address being all 1s.
On an Ethernet LAN the broadcast is set to destination address ffff.ffff.ffff. Point to point interfaces don't have the concept of broadcast, since there is only you and one other endpoint. Broadcasts, at layer 2, are only relevant on Multiaccess networks (Ethernet, Token ring for example).
Layer 3 broadcasts can be all 1s (255.255.255.255) or directed (172.16.255.255). An all 1s broadcast will not be sent off your local segment by the router. A directed broadcast will be sent all the way to the destination network where either it will be dropped there or transmitted on to the wire ("no directed broadcast" on the interface).
If you want some extra reading on the problems with directed broadcast google "Smurf attack".
if I send a request to say 172.16.255.255, will this use ffff.ffff.ffff also in the layer 2 destination mac address ? does the mac address always use ffff.ffff.ffff for both layer 2 and 3 broadcasts, or for directed broadcasts does it use the default gateways destination mac addresss ?
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