Difference Between Broadcast and Multicast

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Broadcasts and multicasts serve different purposes in network communication, particularly in IPv4 and IPv6 environments. Broadcasts send messages to all devices on a network, which can lead to resource overload and inefficiency, while multicasts target specific groups of interested receivers. Multicasts allow for more efficient use of network resources by enabling multiple senders to communicate with only those who have expressed interest in receiving the information. This selective communication is crucial in complex network architectures where security and data classification are important.In Ethernet, broadcasts are identified by a MAC address of all ones (0xFFFFFFFF), while multicasts have a specific format that allows devices to filter messages. IP broadcasts can be system-wide or local, with the former being more dangerous as it can traverse routers. Multicast addresses in IP are defined by a specific range (224.0.0.0 to 239.
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I understand Broadcasts are used in Networks using IPV4 and Multicasts on those using IPV6. I fail to see any advantages of Multicasts over Broadcasts, how subnetting reduces the scope of Broadcasts ( The Broadcast Domain; though I still don't see the difference between Subnets and VLANs too clearly) . I see how Multicasts use up resources and may overhwhelm nodes, but how do Multicasts improve the situation? Sorry, still relatively new to Network Word Salad :).
 
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In multicast, the users of information can send a request to be sent information. They can be checked for their clearance to receive the information. When there are different levels of classification and different "need-to-know" that can be important. Northrop Grumman's Link Management System (LMS) supports protocols that allow unicast, multicast, and other IP protocols. On the other hand, a broadcast sends an open message that everyone can see.
 
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The answer depends on what network protocol you are talking about. Some examples:
Ethernet:
An Ethernet address consisting of all 1's (0xFFFFFFFF) is a broadcast address and is received by all hosts on that network (a broadcast message cannot be blocked).
An Ethernet address with a '1' in the most significant address bit is a multicast address. The hosts on the network can choose whether or not to receive such multicasts (actually, the hardware will let a block of multicast addresses through, it is up to software to filter out unwanted packets).
IP:
An IP address is more complicated than an Ethernet address. Depending on the partitioning into network/host part of the IP address you can have a
  • A system-wide broadcast (IPv4 255.255.255.255) goes to all hosts, including all routers on the network. It passes through the routers and acts as a broadcast on the remote networks. Dangerous!
  • A local broadcast (all 1's in the host part of the IP address) goes to all hosts on the network. It does not pass through routers.
  • There is more...
You can also have IP multicast (the high byte of the IP address = 224). IP defines the concept of multicast groups (which are defined and controlled through the IGMP protocol). Some examples:

224.0.0.0 Base Address (Reserved)
224.0.0.1 All Systems on this Subnet
224.0.0.2 All Routers on this Subnet

224.0.1.10 IETF-1-LOW-AUDIO
224.0.1.11 IETF-1-AUDIO
224.0.1.12 IETF-1-VIDEO
224.0.1.13 IETF-2-LOW-AUDIO
224.0.1.14 IETF-2-AUDIO
224.0.1.15 IETF-2-VIDEO
224.0.1.16 MUSIC-SERVICE

These addresses are listed in the Domain Name Service under MCAST.NET and 224.IN-ADDR.ARPA.
 
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Other than the case of a router not recognizing the MAC address of an incoming message sent to it, what other messages are broadcast/multicast?
 
WWGD said:
Other than the case of a router not recognizing the MAC address of an incoming message sent to it, what other messages are broadcast/multicast?
If a router (or any other host on the network) does not recognize the MAC address, the packet is not received. Examples of MAC broadcast messages are:
Address Type Reservation
FF-FF-FF-FF-FF-FF 0800 IP (e.g. RWHOD via UDP) as needed
FF-FF-FF-FF-FF-FF 0804 CHAOS
FF-FF-FF-FF-FF-FF 0806 ARP (for IP and CHAOS) as needed
FF-FF-FF-FF-FF-FF 0BAD Banyan
FF-FF-FF-FF-FF-FF 8035 Reverse ARP
FF-FF-FF-FF-FF-FF 809B EtherTalk

Examples of MAC multicast addresses are:
01-00-5E-00-00-00- 0800 Internet Multicast [RFC1112]
01-00-5E-7F-FF-FF
01-00-5E-80-00-00- ? Internet reserved by IANA
01-00-5E-FF-FF-FF
01-80-C2-00-00-00 -802- Spanning tree (for bridges)
 
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Svein said:
If a router (or any other host on the network) does not recognize the MAC address, the packet is not received. Examples of MAC broadcast messages are:
Address Type Reservation
FF-FF-FF-FF-FF-FF 0800 IP (e.g. RWHOD via UDP) as needed
FF-FF-FF-FF-FF-FF 0804 CHAOS
FF-FF-FF-FF-FF-FF 0806 ARP (for IP and CHAOS) as needed
FF-FF-FF-FF-FF-FF 0BAD Banyan
FF-FF-FF-FF-FF-FF 8035 Reverse ARP
FF-FF-FF-FF-FF-FF 809B EtherTalk

Examples of MAC multicast addresses are:
01-00-5E-00-00-00- 0800 Internet Multicast [RFC1112]
01-00-5E-7F-FF-FF
01-00-5E-80-00-00- ? Internet reserved by IANA
01-00-5E-FF-FF-FF
01-80-C2-00-00-00 -802- Spanning tree (for bridges)
Apologies, I meant a switch, not a router.
 
WWGD said:
Apologies, I meant a switch, not a router.
A basic Ethernet switch forwards unknown addresses, broadcasts and multicasts to every output (except the port where the message arrived).
In addition, there exists Ethernet switches that understands some IP protocolss (for example IGMP).
 
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Svein said:
A basic Ethernet switch forwards unknown addresses, broadcasts and multicasts to every output (except the port where the message arrived).
In addition, there exists Ethernet switches that understands some IP protocolss (for example IGMP).
Thanks, Svein: If I wanted to see the contents of a multicast, I could just use Wireshark and look up messages by an IP address starting in 224?
 
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WWGD said:
If I wanted to see the contents of a multicast, I could just use Wireshark and look up messages by an IP address starting in 224?
Yes. Or Ethernet addresses of the format 1.0.94.xx.yy.zz.
 
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