![]() When the server sends the multicast traffic to a multicast group with a destination layer 3 address as Class D multicast IPv4 address, it should use destination layer 2 address as a related (mapped) multicast MAC address. Here in multicast, the local multicast server and two clients must agree on a common destination MAC address. You can see that from below image, server is sending multicast packets with destination layer 3 multicast address as 239.10.10.10 and destination layer 2 multicast address as 01:00:5e:0a:0a:0a. But, if the server sends two packets of same data with destination MAC addresses as the MAC addresses of two computers, it is not multicast but unicast. Both computer’s network interface has their own burned-in MAC addresses. Two computers with IPv4 addresses 192.168.10.100 and 192.168.10.103 are interested in receiving multicast traffic sent to multicast group 239.10.10.10. A local server is sending multicast traffic to multicast group 239.10.10.10. To undestand the concept of multicast IPv4 address to MAC address mapping more clearly, consider following scenario. Hence, there should be some mapping between IPv4 multicast addresses and IPv4 multicast MAC addresses. So, the destination MAC address of multicast traffic cannot be the burned-in MAC address of the network interface of an individual computer. But the multicast traffic is sent to a group of computers. Usually the destination MAC address for unicast type of network traffic is the burned-in MAC address of the receiving computer’s network interface. When an IPv4 datagram are sent as unicast, the Ethernet frame which is encapsulating the IPv4 datagram contains sender and receiver MAC addresses as the source and destination MAC addresses respectively. ![]() Address information at IPv4 datagram is source and destination IPv4 addresses, but the address information at layer 2 frame is source and destination MAC addresses. When a unicast data packet is sent to the network, the layer 3 (network layer) IPv4 datagram is encapsulated within layer 2 (datalink layer) frame. Why we need multicast IPv4 address to multicast MAC address mapping
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