The visual compares unicasting and multicasting in IP networks. The top diagram shows a server sending a unicast message to four clients. Note that the server must prepare four copies of the message to send. Each copy is addressed using the individual station address (i.e., the unicast address) of the target client. All four copies of the message are sent in separate IP packets to their destinations.
The bottom diagram shows the use of multicasting in the same setting. The server sends one copy of the message to its attached router. The router functions as a multicast server (MCS), copying the packet the appropriate number of times (in our example, twice) and forwarding the copies toward the target clients. Each router/MCS in the path between the source and the clients repeats the process.
From the example, you can see that the use of multicasting decreases the overall traffic on the network. The source does not have to make or send copies of packets. The number of packet copies multiplies as the target clients are approached.