Multicast can be defined, loosely, as the ability to logically connect a group of hosts in a network in order that they perform many-to-many communication. This group of hosts is called a multicast group or a host group. In a an IP network, multicast is the process whereby a source host or protocol entity sends a packet to multiple destinations simultaneously using a single ‘transit’ operation which implies that the packet transit only takes place once from sender to all destinations in the group rather than once for each destination. The connectionless nature of packet switched network means that the packet sender is not necessarily in the multicast group. A packet switched network is said to provide a multicast service if it can deliver a packet to a set of destinations (a multicast group), rather than to just a single destination. Basically, a multicast service can offer many benefits to network applications in terms of reducing the transmission overhead on the sender, reducing the overhead on the network and time taken for all destinations to receive all the information when an application must send the same information to more than one destinations. The key to efficient multicast is to optimise the duplication of the transmitted data in some sense. Normally, this means keeping the duplication of the transmitted information to a minimum.
	IP multicast uses Class D IP addresses in the range 224.0.0.0 – l 239.255.255.255. These addresses do not identify a single host interface as unicast IP addresses do, but a group of hosts that may be widely, geographically dispersed. This means that special routing procedures are required in the wide-area to enable multicast connectivity. Some of these are reserved, e.g. 224.0.0.1 is the “all systems” address which all hosts must listen to. To contain the scope of IP multicast packets, the TTL field in the IP header is used to limit the maximum number router hops that a multicast packet can traverse before it should be silently discarded.
	Multicast has many benefits over unicast communication in certain areas, e.g. conferencing, software distribution/updates and news distribution. To enable multicast communication, support is needed in the end-systems (hosts and LANs) as well as in the wide-area Internet.