Mapping of IP Multicast Addresses to Ethernet/FDDI addresses.

Both Ethernet and FDDI frames have a 48 bit destination address field. In order to avoid a kind of multicast ARP to map multicast IP addresses to ethernet/FDDI ones, the IANA reserved a range of addresses for multicast: every ethernet/FDDI frame with its destination in the range 01-00-5e-00-00-00 to 01-00-5e-ff-ff-ff (hex) contains data for a multicast group. The prefix 01-00-5e identifies the frame as multicast, the next bit is always 0 and so only 23 bits are left to the multicast address. As IP multicast groups are 28 bits long, the mapping can not be one-to-one. Only the 23 least significant bits of the IP multicast group are placed in the frame. The remaining 5 high-order bits are ignored, resulting in 32 different multicast groups being mapped to the same ethernet/FDDI address. This means that the ethernet layer acts as an imperfect filter, and the IP layer will have to decide whether to accept the datagrams the data-link layer passed to it. The IP layer acts as a definitive perfect filter.Full details on IP Multicasting over FDDI are given in RFC 1390: "Transmission of IP and ARP over FDDI Networks". For more information on mapping IP Multicast addresses to ethernet ones, you may consult draft-ietf-mboned-intro-multicast-03.txt: "Introduction to IP Multicast Routing".If you are interested in IP Multicasting over Token-Ring Local Area Networks, see RFC 1469 for details.Next Previous Contents