Ethernet Multicast

Another category of Ethernet addresses is multicast, used to transmit to a set of stations; streaming video to multiple simultaneous viewers might use Ethernet multicast. The lowest-order bit in the first byte of an address indicates whether the address is physical or multicast. To receive packets addressed to a given multicast address, the host must inform its network interface that it wishes to do so; once this is done, any arriving packets addressed to that multicast address are forwarded to the host. The set of subscribers to a given multicast address may be called a multicast group. While higher-level protocols might prefer that the subscribing host also notifies some other host, eg the sender, this is not required, although that might be the easiest way to learn the multicast address involved. If several hosts subscribe to the same multicast address, then each will receive a copy of each multicast packet transmitted.

We are now able to list all cases in which a network interface forwards a received packet up to its attached host:

•         if the destination address of the received packet maches the physical address of the interface

•         if the destination address of the received packet is the broadcast address

•         if the interface is in promiscuous mode

•         if the destination address of the received packet is a multicast address and the host has told the network interface to accept packets sent to that multicast address

If switches (below) are involved, they must normally forward multicast packets on all outbound links, exactly as they do for broadcast packets; switches have no obvious way of telling where multicast subscribers might be. To avoid this, some switches do try to engage in some form of multicast filtering, sometimes by snooping on higher-layer multicast protocols. Multicast Ethernet is seldom used by IPv4, but plays a larger role in IPv6 configuration.