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Computer Networking: What is the difference between broadcast and flooding?


Simply At the data link level (ISO level 2), a “broadcast” frame is a frame that is intended by its source to be transmitted to every member of the broadcast domain. It is the intention of the frame’s sender that every host in that broadcast domain receive a copy of that frame. This is in contrast to “unicast frames”, which are intended to be received by only one other host in the broadcast domain, and “multicast frames”, which are intended to be received by some, likely more than one, but not all, hosts in the broadcast domain.

Flooding is a mechanism used by Ethernet switches to forward unicast frames to their destination host when the switch doesn’t know which of its several ports that particular destination is on. Ethernet’s original design assumed that all of the members of the Ethernet broadcast domain were continuously electrically connected, and so any frame sent by one member would be seen by every other member, whether or not the receiving members were “interested” in that frame (that is, it was addressed to them or sent to the broadcast frame address of all-ones). This made sense when the shared-bus topology of a tapped coaxial cable was being used. But the introduction of star hubs (StarLAN for the win!) and twisted-pair connections over Category 3 (and later Category 5 and 6) cable made it practical for hub devices to become switches, forwarding frames that arrive on one port to only the single other port where the destination of that frame is known to be. This increases the total available bandwidth of the network and prevents one conversation between two hosts from consuming the entire bandwidth of the network. If the switch can’t tell which of its ports a given frame needs to go (because it hasn’t learned the port that has the device with that MAC address), it floods the frame to every port (except, of course, the one it came in on), in the hopes that the desired target is out there somewhere.

Multicast frames can be selectively forwarded as well if the switch has the appropriate intelligence to learn which of its ports participate in a given IGMP multicast group; otherwise, they’re also flooded.  Broadcast frames are, of course, always flooded.

Above One Is Answer From Book – Thinking In Java. Now Understand It Better In My Way.

Broadcast is the name we have assigned to some particular frames i mean the frames which have destination addresses ff-ff-ff-ff-ff-ff (MAC for layer 2 broadcast) or (IP for layer 3 broadcasts).

In networking broadcasting refers to transmitting a packet that will be received by every device on the network. Read carefully it means the device generates and transmits a frame which will be received by each device (in it’s broadcast domain).

Now i hope you know basics about a switch. A switch floods a frame if it cannot find the destination address in it’s MAC lookup table. So flooding here is sending the frame to all the devices connected to ports because the exact port for the destination address is not known.

Now understand it like this suppose the device at port no. 4 wanted to converse with device with MAC 00 34 d4 fs 45 a7 (assume anything that is not already in the table) so when it will forward the frame to the switch the switch will look in the MAC table and will forward it to every device connected on its ports (there would be many devices it has not acknowledged the port no. of). The device may or may not exist in broadcast domain of switch (who knows). Flooding occured here as it sent the frame to all.

Now lets come to broadcast. The device at port 4 sends a broadcast message (see carefully the device 4 is broadcasting ) that is transmits a message which is supposed to be recieved by all the devices that is it mentions the destination address as ff-ff-ff-ff-ff-ff . Now the switch receives the packet. It will now check if this destination address has some mapping (with some port) in it’s lookup table. And it doesn’t have. So now it will Flood all it’s ports where devices are connected.


The thing that you need to understand here is the switch floods because it could not find that address in it’s lookup and not because it was asked to send to all.

See “Let me send it to all because it’s a broadcast message and so should be sent to all” and “Let me send it to all because this MAC is not in my table” aren’t exactly same things. They do have the same impacts though. So for switch both the MAC addresses ff-ff-ff-ff-ff-ff and 00 34 d4 fs 45 a7 mean the same (because both of them don’t exist in it’s MAC table). So for switch the broadcast frame is a subset of so many frames that would lead it to flood it’s ports (means all the frames possible whose destination MAC it doesn’t have in it’s table).

The switch itself can’t generate a broadcast message (the way device 4 did) it can just flood if it receives some broadcast frame.

Also do understand that in cybersecurity you use flooding to point out the malicious behaviour while broadcast for the opposite. I wrote the answer considering it to be computer networking question and not of cybersecurity or hacking.

Have something to say about this article? Comment below or share it with us on Facebook or our WhatsappGroup.

Lovepreet Singh

Privacy Policy – River VPN

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