The broadcast systems generally allow the possibility of addressing a packet to all destinations by using a special code in the address field. When a packet with this code is transmitted then it is received and processed by every machine on the network. This mode is called broadcasting.
Some broadcast systems also support transmission to a subnet of the machines. This is known as multicasting. In Other words, Multicast is a communication system between a single sender and multiple receivers on a network. One address bit is reserved for multicasting and the remaining (n - 1) address bits can hold a group number. Any machine can subscribe to any or all of the groups. The IP (Internet Protocol) address supports concept of classes. This architecture is called Classful addressing. In Classful addressing, the IP address space is divided into five classes: classes A, B, C, D and E. Each class occupies some part of the whole address space.
• Addresses in class D are for multicast communication from one source to a group of destinations.
• If a host belongs to a group or groups it may have one or more multicast addresses.
• A multicast address can be used only as a destination address but never as a source address.
• Unicast communication is one source sending a packet to one destination.
• But multicast communication is one source sending a packet to multiple destinations.
• Such a communication takes place using multicast packets.
Multicasting has many applications such as access to distributed database, information dissemination, distance learning and multimedia communications.
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