Hi kids! I'm Sauron and this year I'm going to use Christmas and the network simulator that Jorge is developing to show you the difference between Broadcast, Unicast and what is NAT in a router.
If you press the button you will see that a computer in network A sends a message to all the hosts in that network. That's a broadcast. It's like yelling "Merry Christmas!" to everyone at home. Take into account that the router does not retransmit the broadcast, but the switches do.
The button sends the message to only one computer in network A. Something similar to going to your son's room and wish him, and only him, Merry Christmas.
And what if we want to wish Merry Christmas to someone living in some other house? In this case, we will use the router's public IP address with a unicast message using a message. The router with NAT enabled sends the message to a host in the network, depending on the destination port. It is like calling by phone to some other house, and asking by someone specifically.
Uhhhmmm... why was I telling this?... oh, yes... Merry Christmas and happy New Year to everyone!. Be good and try making 2015 a better year than 2014, because the world needs it.