eli5:what is broadcast address how is it used in layer 2 and layer 3 of osi model?
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The last address of a given address range is usually reserved as the “broadcast” address. Any message sent to that broadcast address will be forwarded to all devices within that address block.
A broadcast address is the last legal address within a subnet and it is used to send a packet to all IPs within that subnet.
Imagine you live in a house on a Baker street. That house will have a mailing address, and that mailing address works very similarly to an IP address. You use it to send and receive packets to the right house.
Imagine the Post office had a mechanism for flyers where if you sent a flyer to 9999 Baker street, the post man would deliver a copy of that flyer to every house on the street. That’s basically how a broadcast address works.
A broadcast address is the address that you send something to when you don’t want to reach a specific recipient but instead everyone.
You can think of it like the difference between a phone call and a radio broadcast. With your phone you call the number of a specific recipient to talk to them, with a radio broadcast you just send your message out to anyone who may be listening.
A Layer two broadcast send to a special MAC address that is all 1s and no 0s. FF-FF-FF-FF-FF in hex.
A Layer three broadcast is sent to the IP Address that is the last IP Address in the network. It takes the form of whatever your Network address is plus a all ones for the rest.
For example if your network is 192.168.1.0/24 the broadcast address will be 192.168.1.255.
One example of when you might use a broadcast is when you don’t have an IP address yet and don’t know anything about the network. A new computer connected to a network will typically send out a broadcast to the entire Layer 2 network it is in in hopes of getting an answer from a DHCP server that will tell it what IP address to use what network it is in and where the exits are and a bunch of other stuff about DNS and whatever.
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