Difference Between Broadcast and Multicast

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Discussion Overview

The discussion revolves around the differences between broadcast and multicast in networking, particularly focusing on their applications in IPv4 and IPv6. Participants explore the advantages and disadvantages of each method, the implications of subnetting, and the technical details of how these methods operate within network protocols.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that broadcasts are used in IPv4 networks while multicasts are associated with IPv6, expressing confusion about the advantages of multicasts over broadcasts.
  • Another participant explains that broadcasts have one sender, whereas multicasts can involve multiple senders, allowing messages to be sent only to interested parties.
  • A contribution highlights that multicast allows users to request information based on their clearance levels, contrasting it with the open nature of broadcasts.
  • Technical details are provided regarding Ethernet and IP addresses, explaining how broadcasts and multicasts are defined and processed differently in these contexts.
  • Examples of MAC broadcast and multicast addresses are shared, illustrating the types of messages associated with each method.
  • Some participants clarify the role of Ethernet switches in forwarding broadcasts and multicasts, noting that switches can handle unknown addresses differently.
  • There is a suggestion that tools like Wireshark can be used to monitor multicast messages by filtering for specific IP address ranges.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and agreement on the advantages of multicasts compared to broadcasts. Some technical details are clarified, but no consensus is reached on the overall superiority or utility of one method over the other.

Contextual Notes

The discussion includes technical jargon and assumptions about networking protocols that may not be universally understood. There are unresolved questions regarding the practical implications of using broadcasts versus multicasts in different scenarios.

Who May Find This Useful

This discussion may be useful for individuals interested in networking concepts, particularly those exploring the differences between broadcast and multicast methods in various network protocols.

WWGD
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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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