When using unicast method, one device will send the message to exactly one destination device. If some device needs to send a message to multiple devices, it will have to send multiple unicast messages, each message addressed to a specific device. So, the sender has to send a separate message to each destination device, and to do that it has to know the exact IP address of each destination device. Remember that in unicasting, each packet is destined for only one device.

Source: http://www.utilizewindows.com/the-difference-between-unicast-multicast-and-broadcast-messages/

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