PyNet Labs- Network Automation Specialists

Multicast Communication

Last Updated : December 13, 2022
What is Multicast Communication

Before starting about multicast, we have to understand why we need multicast communication and what possible challenges we can face without multicast communication. In this blog, we will cover how multicasting works and how the data is processed in multicast communication, its advantages, multicast forwarding, and everything you need to know about it.

Key Takeaways -

The term "multicast" refers to a method of sending a single message to a large group of people. Multicast Communication is a type of technique that transfers packets from one source to many receivers simultaneously.

Multicast relies on three technologies namely - IGMP, PIM, Layer 3 Multicast.

Multiple devices can now communicate with one another across the globe because of the interconnectivity made possible by computer networks. This kind of interaction, however, is not easy and requires a wide range of technological resources, from the physical to the logical, to take place.

Cables, intermediate devices, addressing schemes, and protocols are all examples of such resources. More specifically, researching and constructing communication systems that ensure messages are addressed correctly is essential. Effective communication between linked devices may be ensured if these techniques are widely used. However, its incorrect use might cause network congestion and safety issues.

Before jumping to what is Multicast communication, let’s first look at communication and it various types.

Communication and Its Types

In computer networks, "casting" refers to the process of sending information from one or more clients to one or more recipients across a shared communication medium called a network. Computer networks use three distinct forms of casting:


If you send the traffic from one source to one destination, it is unicast traffic. Whereas if we are sending from one source to all, it is considered as broadcast.

And about multicast, we all know that it’s used to send data from one source to a group of receivers.

Almost all of the data exchanged in an IP network is of the directed unicast type. By definition, only the two hosts involved in the initial request may interact with one another through the path set up by the unicast.

Watch this video to know more about Types of Communication –

Due to the high cost of transmitting a single audio or video stream to a single user, unicasting such streams to a wide audience is impractical. Therefore, this is an ideal scenario for multicast communication since it allows sharing of a single stream to multiple hosts.

Message Addressing Method

Now that we have good idea of communication, its time to move to Multicast Communication.

What is Multicast Communication?

The term "multicast" refers to a method of sending a single message to a large group of people and a tool to make the most of available network bandwidth while conserving system resources. Also, we can say that Multicast communication is a type of technique that transfers packets from one source to many receivers simultaneously.

We also use many applications in our daily life, like Audio/Video Conferencing, Online Gaming, IPTV, etc. The best part is that all these applications work on multicast communication.

Multicast Communication basic terminologies

There are a few important terms in Multicast communication and these are very basic terminologies that are known by different names in normal.

  • In multicast communication, the sequence of data packets is called a stream.
  • Multicast destination IP address is considered a group address.
  • Recipient devices of a multicast stream are generally referred to as receivers.
Multicast Communication

Let's move on and understand the advantages of Multicast now.

Advantages of Multicast

The success of certain cutting-edge applications is greatly aided by the benefits offered by multicast technology. Several of these advantages of Multicast are discussed below.

  • Better bandwidth utilization
  • Better resource utilization at the source
  • Better resource utilization at the receivers
  • The one-to-many communication provided by multicast traffic is achieved by sending just a single data packet over a connection and then replicating it across other links at the request of recipients.

These are the most common advantages of Multicast.

NOTE: Another way of providing one-to-many packet delivery is Replicated Unicast.

Let’s see a quick comparison between the two.

Multicast Vs. Replicated Unicast

MulticastReplicated Unicast
In multicast source sends only a single copy of any packet.In replicated unicast, the source would have to send as many copies as the number of known receivers.

Multicasting Technology

In this section, we will discuss various multicasting technologies. Each of the following multicasting technologies has its place in certain contexts and with certain applications. Still, Multicast communication relies on the following:


Network multicasting is managed and controlled by the Internet group management protocol (IGMP). It's not limited to only LANs but also WANs. IGMP for its operation in L2 networks.

IGMP Versions

There are 3 versions available –

a. IGMP v1 – old version not used today.

b. IGMP v2 – common in most multicast networks.

c. IGMP v3 – used by source-specific multicast

IGMP Versions

IGMP v1 & v2 Messages

  • Membership Report (from receivers to MC-enabled router – for joining an MC group)
  • Leave Group (from receivers to MC-enabled router – for leaving an MC group – only in V2)
  • Membership query (IGMP Hello – from MC enabled router to MC receivers.

2. PIM

PIM is a mechanism for sending multicast data packets across a network. A variety of unicast routing protocols may be used by PIM to determine the best route between sender and recipient. PIM's main benefit over IGMP is that it reduces end-to-end latency in a multicast session by eliminating the need to monitor the receiver status. PIM allows it to function on Layer 3 networks.

PIM Operating Modes –

a. PIM Dense Mode (PIM-DM)

b. PIM Sparse Mode (PIM-SM)

c. PIM Sparse Dense Mode

d. PIM Source-Specific Multicast (PIM-SSM)

e. PIM Bidirectional Mode (Bidir-PIM)

3. Layer-3 Multicast – IP Addressing

In L-3 Multicast - IP Addressing, Source IPs in multicast packets always remain in the form of unicast, whereas the destination IPs represent the multicast group. And these destination Multicast IPs belong to the D-Class of IPv4 address.

The first four bits of this whole range start with 1110.

And this is the entire range for Multicast Address - -, and every single IP simply represents a multicast group.

Layer 3 Multicast MAC

Source IPs in multicast packets always remain in the form of unicast as we know that there is a one-to-one correlation between IP and MAC at a data-link layer where we have two sub-layers of Data-Link, which are MAC & LLC.

But when it comes to destination IP, a specific MAC is reserved for multicast destination IP. The motive is to have the sender and receiver agree on a single, suitable destination MAC.

So, a new special reserved MAC is created to send multicast packets in the IP networks so that multicast traffic replication doesn't have packet manipulation and the burden on routing resources could be avoided.

The IP address of a multicast group is translated to a unique MAC address, which is used by interfaces to determine which multicast packets belong to which reserved groups. Depending on the MAC address associated with the multicast traffic, a LAN segment can support multiple streams, and the receiver can choose which stream to forward to the CPU for processing. So, that’s how the mapping is done here with the help of this reserved MAC add which is attached to multicast IPs.

NOTE: Multicast Reserve MAC Address always starts with 01:00:5E

Reserved Multicast Addresses

Reserved Multicast Addresses

Mapping L3 TO L2

To provide this, a simple mapping procedure is used to generate a well-known multicast MAC address which is the special MAC Address (01:00:5E)

  1. The first 24 bits of a multicast MAC address always start with 01:00:5E.
  2. The 25th bit is always 0.
  3. A multicast MAC address is created by duplicating the lower 23 bits of a multicast group IP address.

To illustrate this point, let's presume that we have the multicast address Mappingthe first 24-bit standard part will be – 01:00:5E (Reserve MAC). Also, the 25th Bit is always set to - 0.

01:00:5E – 0000|0001: 0000|0000: 0101|1110

(This is the conversion of special MAC)

Now we have this multicast address – so, we’ll convert the remaining 24 bits of this IP. – 11100000.00000000.00000000.00000101

(This is the conversion Multicast IP) so, now we’ll use the remaining 24 bits – 11100000.00000000.00000000.00000101

So, the last 24 bits will be - 00000000.00000000.00000101

The bold and italic mark is the special 25th bit which is always 0

Now, the resulting destination MAC will be – 01:00:5E:00:00:05

Let's move on and understand Multicast forwarding.

Multicast Forwarding

A collection of hosts is often clustered under a multicast IP address in multicast forwarding. Data may then be sent from the source to the IP address, which subsequently sends data to the many hosts allocated to the multicast IP. This is a data transfer from one location to multiple locations.

  • In unicast forwarding, the packets are always guided toward the destination.
  • In multicast forwarding, the packets are always guided away from a source.

There are two major interface types per multicast source -

  1. Upstream interface - closest to the source
  2. Downstream interfaces – are considered receivers

So, we call this entire concept reverse path forwarding (RPF).

If you want more insights about Multicast Communication, you can also check out this video –

We have now taken a closer look at everything related to Multicast Communication. If you are still with us, it would be great if you share your honest feedback about this article. If you like this article, subscribe to our free newsletter.

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Versa SD-WAN Certification

Multicast is an integral part of PyNet Labs' CCNA training. You should check out our training by clicking this link -

To conclude this article, we can say that communication methods represent vast opportunities for developing networked systems. With the proper software engineering and correct practical use of addressing methods, systems can communicate in the network with improved efficiency, reliability, and security.

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