The majority of the traffic on the Internet is unicast: one source device sending data to one destination device. However, it is possible to have one source device sending the same data to a group of devices; this is called multicasting or a multicast service. The Internet Protocol (IP) also supports multicasting. This service type can be considered a more efficient alternative to broadcasting because only selected hosts can be designated receivers.


7.1.1 Multicast Addressing

The classful IP addressing scheme in IP version 4 (IPv4) sets aside 1/16 of the address space for multicast addresses; this group of addresses is known as Class D. Multicast addresses are identified by the pattern 1110 in the first four bits, which corresponds to a first octet of between 224 and 239. The full range of multicast addresses is from to Since multicast addresses represent a group of IP hosts (or interfaces), which is sometimes called a host group, they can only be used as the destination of a multicast datagram and never as the source.

Under IPv4, this block of addresses is defined with this objective and covers several multicast services and applications [1]. This large multicast class is subdivided into blocks according to how far the packets are allowed to travel and the type of services provided. The address can be combined with specific values of the time-to-live (TTL) to limit the distance (in the number of routers) that ...

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