Legacy Document

Inside Macintosh: Networking /

Chapter 11 - Ethernet, Token Ring, and Fiber Distributed Data Interface / About Ethernet, Token Ring, and FDDI Support

About Multicast Addressing

At the hardware device driver level, Apple supports multicast addressing. A multicast address is a hardware address that is shared by a subset of nodes on a particular data link. This is similar in concept to a broadcast hardware address, but a multicast address is used to send directed broadcasts to the subset group of nodes only, and not to all nodes on the data link. A broadcast address is shared by all nodes on a particular type of network. Packets sent to the Ethernet broadcast address are sent to all nodes on the Ethernet data link. Ethernet and FDDI networks use multicast addresses; the token ring equivalent of a multicast address is a functional address.

A network type, such as an Ethernet data link, can also have associated with it one or more multicast addresses. Each data link is identified by a unique hardware address to which packets for that network hardware are sent. In addition to this unique hardware address, a data link can receive packets that contain the broadcast address for its own network type--Ethernet, for example.

When a node on a data link transmits a packet that has a multicast hardware address as its destination hardware address, then only a specific subset of the nodes on the link will receive the packet. Each node can have any number of multicast addresses, and any number of nodes can have the same multicast address. Some nodes on the link may not have a multicast address; other nodes may have more than one multicast addresses. (For more information on multicast and functional addresses, see Inside AppleTalk, second edition. See also "EAddMulti" on page 11-40.)