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The great bulk of TCP/IP communications uses the Internet Protocol to send messages from one source device to one recipient device; this is called unicast communications. This is the type of messaging we normally use TCP/IP for; when you use the Internet you are using unicast for pretty much everything. For this reason, most of my discussion of IP has been oriented around describing unicast messaging.
IP does, however, also support the ability to have one device send a message to a set of recipients. This is called multicasting. IP multicasting has been officially supported since IPv4 was first defined, but has not seen widespread use over the years, due largely to lack of support for multicasting in many hardware devices. Interest in multicasting has increased in recent years, and support for multicasting was made a standard part of the next generation IP version 6 protocol. Therefore, I felt it worthwhile to provide a brief overview of IP multicasting. It's a large and very complex subject, so I will not be getting into it in detailyou'll have to look elsewhere for a full description of IP multicasting. (Sorry, it was either a brief summary or nothing; maybe I'll write more on multicasting in the future.)
The idea behind IP multicasting is to allow a device on an IP internetwork to send datagrams not to just one recipient but to an arbitrary collection of other devices. IP multicasting is modeled after the similar function used in the data link layer to allow a single hardware device to send to various members of a group. Multicasting is relatively easy at the data link layer, however, because all the devices can communicate directly. In contrast, at the network layer, we are connecting together devices that may be quite far away from each other, and must route datagrams between these different networks. This necessarily complicates multicasting when done using IP (except in the special case where we use IP multicasting only between devices on the same data link layer network.)
There are three primary functions that must be performed to implement IP multicasting: addressing, group management, and datagram processing / routing.
Special addressing must be used for multicasting. These multicast addresses identify not single devices but rather multicast groups of devices that listen for certain datagrams sent to them. In IPv4, 1/16th of the entire address space was set aside for multicast addresses: the Class D block of the original classful addressing scheme.
Special techniques are used to define the meaning of addresses within this block, and to define a mapping between IP multicast and data link layer multicast addresses. This is all described in the topic on IP multicast addressing; mapping of IP multicast addresses to hardware layer multicast addresses is discussed in the section on address resolution.
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