The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

10.1. Multicast Usage

Multicast is really a wonderful and powerful mechanism. Networks have always incorporated the concept of station-to-station (unicast) addressing, and most multipoint networks also provide a means to send data to all stations at once (broadcast). Multicast is an important generalization of the multiple-recipient delivery concept; it provides the ability to send frames to any arbitrary set of devices.

Multicast, as it is currently used, was developed as an integral part of the original Ethernet system. A key enabler was the decision to use a globally administered 48-bit address space. Generalized multicast addressing greatly increases the number of addresses needed. For a catenet of n devices, only n unicast addresses are needed (one per device), but the number of possible multicast groupings of n devices grows exponentially large. In a catenet comprising n devices, the number of possible distinct multicast groupings is:

While in practice you never need quite that many distinct groupings, the use of multicast addressing generally requires that you provide for more unique addresses than there are devices in the catenet.^[] However, because you provisioned an address space with 2⁴⁸ elements, it is considered unnecessary to carefully conserve addresses. You split the address space in half — half for unicast and half for multicast. Unicast addresses denote a physical ...