The other three flags are known as R , P and T. Similar to a unicast address, the prefix of an IPv6 multicast address specifies its scope, however, the set of possible scopes for a multicast address is different.
MAC Address Mapping > Network Access and Layer 2 Multicast
The 4-bit sc or scope field bits 12 to 15 is used to indicate where the address is valid and unique. The service is identified in the bit Group ID field. For example, if ff The Group ID field may be further divided for special multicast address types. Ethernet frames with a value of 1 in the least-significant bit of the first octet [note 3] of the destination MAC address are treated as multicast frames and are flooded to all points on the network. Modern Ethernet controllers filter received packets to reduce CPU load, by looking up the hash of a multicast destination address in a table, initialized by software, which controls whether a multicast packet is dropped or fully received.
From Wikipedia, the free encyclopedia.
A multicast is indicated by the first transmitted bit of the destination address being 1. CS1 maint: Multiple names: Multicast Tech. Archived from the original on IPv6 Essentials Second ed. Internet Assigned Numbers Authority. Karl Auerbach.
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MAC Address Mapping
This chapter covers the basic elements of multicast functionality in Layer 2 domains as well as design considerations for multicast deployments. IP Multicast, Volume I: Cisco IP Multicast Networking. A traditional Ethernet switch Layer 2 device works with Ethernet frames, and a traditional router Layer 3 device looks at packets to make decisions on how messages will be handled.
As discussed in Chapter 1, when a device sends a broadcast frame, the destination address is all ones, and a unicast message is the destination MAC address. What happens when it is a multicast message? To optimize network resources, an Ethernet switch also needs to understand multicast. This is where the magic happens. The sending device must convert the destination IP multicast address into a special MAC address as follows:. The high-order 25 bits is the official reserved multicast MAC address range from The MAC Addresses page is available to help you learn more about them.
A multicast is similar to a broadcast in the sense that its target is a number of machines on a network, but not all. Where a broadcast is directed to all hosts on the network, a multicast is directed to a group of hosts. The hosts can choose whether they wish to participate in the multicast group often done with the Internet Group Management Protocol , whereas in a broadcast, all hosts are part of the broadcast group whether they like it or not!
Multicast - Understand How IP Multicast Works
As you are aware, each host on an Ethernet network has a unique MAC address, so here's the million dollar question: How do you talk to a group of hosts our multicast group , where each host has a different MAC address, and at the same time ensure that the other hosts, which are not part of the multicast group, don't process the information? You will soon know exactly how all this works.
In order to explain Multicasting the best I can and to make it easier for you understand, I decided to break it down into 3 sections:. The brief diagram below shows you the relationship between the 3 and how they complete the multicasting model:. When a computer joins a multicast group, it needs to be able to distinguish between normal unicasts which are packets directed to one computer or one MAC address and multicasts.
With hardware multicasting, the network card is configured, via its drivers, to watch out for particular MAC addresses in this case, multicast MAC addresses apart from its own. When the network card picks up a packet which has a destination MAC that matches any of the multicast MAC addresses, it will pass it to the upper layers for further processing.
Ethernet uses the low-order bit of the high-order octet to distinguish conventional unicast addresses from multicast addresses. To understand this, we need to analyse the destination MAC address of a unicast and multicast packet, so you can see what we are talking about:. The following picture is an example of my workstation Now let's analyse the destination MAC address: When my gateway receives the packet, it knows it's a unicast packet as explained in the above picture.
Let's now have a look at the MAC address of a multicast packet.
Multicast IP Address to MAC address mapping
Keep in mind, a multicast packet is not directed to one host but a number of hosts, so the destination MAC address will not match the unique MAC address of any computer, but the computers which are part of the multicast group will recognise the destination MAC address and accept it for processing.
The following multicast packet was sent from my NetWare server. Notice the destination MAC address it's a multicast:. Analysis of a multicast destination MAC address:
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