• A message can be unicast, multicast, or broadcast. Let us clarify these terms as they relate to the Internet.

Unicasting

• In unicast communication, there is one source and one destination. The relationship between the source and the destination is one-to-one.

• In this type of communication, both the source and destination addresses, in the IP datagram, are the unicast addresses assigned to the hosts (or host interfaces, to be more exact).

• In Figure 1 a unicast packet staI1s from the source S1 and passes through routers to reach the destination D1. We have shown the networks as a link between the routers to simplify the figure.

• Note that in unicasting, when a router receives a packet, it forwards the packet through only one of its interfaces (the one belonging to the optimum path) as defined in the routing table.

• The router may discard the packet if it cannot find the destination address in its routing table.

In unicasting, the router forwards the received packet through only one of its interfaces.

Multicasting

• In multicast communication, there is one source and a group of destinations. The relationship is one-to-many.

• In this type of communication, the source address is a unicast address, but the destination address is a group address, which defines one or more destinations.

• The group address identifies the members of the group. Figure 2 shows the idea behind multicasting.

• A multicast packet starts from the source S1 and goes to all destinations that belong to group G1. In multicasting, when a router receives a packet, it may forward it through several of its interfaces.

Broadcasting

• In broadcast communication, the relationship between the source and the destination is one-to-all. There is only one source, but all the other hosts are the destinations.

• The Internet does not explicitly support broadcasting because of the huge amount of traffic it would create and because of the bandwidth it would need.

• Imagine the traffic generated in the Internet if one person wanted to send a message to everyone else connected to the Internet.