332 C HAPTER 13: IP MULTICAST R OUTINGIP MulticastOverviewThe easiest way to begin to understand multicasting is to compare itagainst two other address types and their communication models.Unicast Model A unicast address is designed to transmit a packet from a source to asingle destination. Unicast transmissions are for one-to-onecommunication. If multiple users need to receive the samecommunication, the source operating in unicast mode generates andsends each copy separately.Broadcast Model A broadcast address is used to send a datagram from a source to multipledestinations — an entire subnetwork, for example. Broadcasttransmissions produce one-to-many communication, but some of thereceivers may not want or need to receive the communication.Multicast Model A multicast address is used for one-to-many and many-to-manycommunication in an environment where users and network deviceseither explicitly or implicitly communicate their desire to receive thecommunication.In contrast to unicast, a source that uses IP multicast generates and sendsonly one copy of the information that is desired by multiple receivers. Atpoint where the delivery path that reaches group members diverges,network devices replicate and forward the packets. This approach makesefficient use of both source processing power and network bandwidth.When using the Internet Protocol (IP) as the basis for multicastcommunication, the requests for and delivery of the communication isfundamentally controlled by referencing certain IP addresses or theirMAC-based equivalents. These addresses are called group addresses orgroups and hosts that reference these addresses are called groupmembers.IP multicast group members can be scattered across multiplesubnetworks; thus, successful transmission from a source to groupmembers can occur within a campus LAN, a MAN, or over a WAN.As an extension to the standard IP network-level protocol, IP multicastwas first defined in 1985 in RFC 966. Certain other protocols are used tosupport IP multicast processes. These are explained later in this chapter.
