This rule is covered in the last section of this page, but you don't need to know it now in order to understand Hardware multicasting. The IEEE group used a special Rule to determine the various MAC addresses that will be considered for multicasting. Multicast MAC addresses are always used in the destination MAC address field of an Ethernet packet. Remember, the destination MAC address 01-00-5E-00-00-05 does not belong to a host, and is recognised by computers that are part of the multicast group. ![]() So now you should be able to understand how computers can differentiate between a normal or unicast packet and a multicast packet. Notice the destination MAC address (it's a multicast):Īnalysis of the multicast destination MAC address will help make things clearer: The following multicast packet was sent from our multicast server. Keep in mind, a multicast packet is not directed to one host but a group 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. Let's now have a look at the destination MAC address of a multicast packet. When my gateway receives the packet, it knows it's a unicast packet as explained in the above image. Now let's analyse the destination MAC address: The following picture is an example of my workstation (192.168.0.6) sending a packet to my network's gateway (192.168.0.5): When a normal (unicast) packet is put on the network by a computer, it contains the Source and Destination MAC address, found in the 2nd Layer of the OSI model. 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: A unicast would have this bit set to ZERO ( 0), whereas a multicast would be set to ONE (1) 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.Įthernet uses the low-order bit of the high-order octet to distinguish conventional unicast addresses from multicast addresses. ![]() 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 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. The brief diagram below shows you the relationship between the 3 and how they complete the multicasting model: Later on I will talk about Mapping IP Multicast to Ethernet Multicast which is really what happens with multicasting on our Ethernet network using the TCP/IP protocol.
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