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Theoretically speaking, as far as I know Ethernet was developed to be used as a broadcasting protocol and technology. I guess nowadays it is not being used this way. Since ALL the examples I see refer to point-to-point communication, I cannot comprehend how it is used for multicast communications.

I know that an example to Ethernet being used in broadcasting is DHCP Discovery message. However, I cannot think of it being used as a means to communication itself.

I can collaborate more if needed. Thank you for your help

  • "Since ALL the examples I see refer to point-to-point communication" You really need to explain that. For example, typical ethernet LANs use switches where there are many hosts on the same LAN, and broadcast is required. Also, multicast is a form of broadcast. – Ron Maupin May 29 '18 at 16:24
  • Even if the typical communication is one host to one host, that doesn't make the underlying network point-to-point. In the case of ethernet, you might care to think of it as any-to-any, though certainly broadcasts are used for lots of protocols, especially those involved in finding hosts or requiring "simultaneous" delivery of the same content. N hosts plugged into a switch really doesn't resemble point-to-point links at all. – jonathanjo May 29 '18 at 16:38
  • Look at PPP (literally, Point-to-Point Protocol). It has no need for addressing because there are only two possible endpoints, so anything sent from one endpoint gets to the only other endpoint - the only possible destination. Ethernet requires addressing (MAC addressing) because it is not point-to-point; there may be many possible destinations on an ethernet LAN, and broadcast is required to discover neighbors on the LAN. – Ron Maupin May 29 '18 at 16:42
  • @RonMaupin I may have used PPP term wrongly, what I meant was any-to-any I guess. The problem is, the definition of broadcast is you send a message and that single message is received by many others and used. Other than the discovery messages on LAN, can you tell me an example in which you send a packet with Ethernet and communicate to several others via this packet. I mean something like a radio station does with transceivers. – Ninja Bug May 29 '18 at 17:29
  • Understand that ethernet is like a multitool. Ethernet really doesn't know about any data or protocols it carries, and it has been used for many different network protocols, each of which can carry any number of transport protocols, which can each carry any number of application protocols. Think of something like IPTV. You can have a sender that sends sound and/or video to many different hosts. Rather than the sender creating a connection for each receiver, it can send to a group. Ethernet addresses have a bit (I/G) determining if the destination address is an individual or group address. – Ron Maupin May 29 '18 at 17:36
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Ethernet was developed to be used as a broadcasting protocol and technology.

That is incorrect. Ethernet was developed for a shared access media -- originally, a physical bus that ran throughout the office, and every host connected to the bus. It could be either unicast, multicast, or broadcast.

Modern networks use switches instead of a bus, but logically, they are the same.

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Initially, Ethernet was based on broadcasting physical layer technology (bus network, repeaters) but logically (layer 2), the protocol has always been MAC-based many-to-many.

Modern networks don't use the broadcasting approach any more (for the past 15+ years) but purely MAC-based packet switching.

Broadcasting and multicasting are done using special addresses that the switches recognize. The broadcast address is all-one FF-FF-FF-FF-FF-FF and multicast addresses have the least significant bit in the first octet set to 1. A broadcast frame is forwarded to all ports (but the one it was received on), multicasts are forwarded to subscribed ports.

Ethernet's (non-ancient) physical layer protocols use point-to-point links, so that there are always exactly two ports in a link.

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Zac (alluded to the I/G bit) and Ron (pointed out that shared medium and broadcast are different things) have given you a couple of perspectives, and I will give you to another perspective by contrasting the switched ethernet broadcast protocol with the switched frame relay non-broadcast protocol.

Both ethernet and frame relay can be switched. Ethernet has broadcast, but frame replay does not. Ethernet switches will find a destination host on the LAN by flooding (if necessary), but frame relay switches must be pre-configured (incoming DLCI, outgoing interface, and outgoing DLCI) to be able to send a frame from one endpoint to another endpoint. Ethernet switches that do not know where a destination host is will flood the frame to all the other switch interfaces, and the correct destination will get the frame, but so will all other hosts on the network. Frame relay does not do that, and it would be very undesirable for a typical use case. Frame relay can also be connected point-to-point, as can ethernet, but frame relay still cannot broadcast or multicast, while ethernet can.

For example, consider OSPF. OSPF considers ethernet a broadcast network, and an OSPF router on an ethernet network will use multicast (one frame sent to reach all the OSPF routers on the network, using a multicast destination address to which all OSPF routers will subscribe) because ethernet supports broadcast and multicast. OSPF considers frame relay to be a non-broadcast network, and the source OSPF router needs to replicate each frame to each OSPF router on the network because frame relay doesn't have broadcast or multicast. Multicast is preferred because it places less burden on the router and network when communicating with multiple peer routers.

Even if you physically connect one OSPF router with a physical point-to-point link to a single other OSPF router using ethernet (with no possibility of any other hosts on the link), OSPF will default to using multicast. You can override that behavior by configuring the OSPF ethernet interface as a point-to-point interface, but that will fail if there are multiple OSPF routers on the ethernet network.


As Ron points out, you seem to be confusing a shared medium (possible to have collisions if two or more hosts simultaneously transmit) with broadcast. The various ethernet standards before 10 Gbps all are written to the possibility that ethernet could be connected to a hub (shared medium) with the possibility of collisions, but that was eliminated at 10 Gbps (the engineers finally bowed to the fact that switched ethernet is the new reality), but broadcast and multicast were not eliminated from the standards because they are still necessary features of ethernet.

As Zac points out, MAC addresses have a couple of special bits. One of the bits (U/L) tells you if the MAC address is a universal address (assigned by the manufacturer), or if the address is locally administered (assigned by the network administrator to override the universal address). The other bit (I/G) tells you if the MAC address is an individual address (assigned to a single host), or if it is a group address (broadcast to all hosts or multicast to hosts subscribing to the multicast group).

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