The key point is that a switch doesn't understand IP addresses, only MAC addresses. Thus the router needs to find the destination MAC with ARP before it sends it to the switch. Then the switch can forward the frame out the right port.

Yes, you'd think an IP address would be enough (Unique), but again switches don't understand IP so we need to find the MAC.


Twilight Sparkle: http://psychicwalnut.deviantart.com/art/Angry-Princess-Twilight-456389112

Fluttershy: http://img14.deviantart.net/d2d8/i/2011/123/2/1/dancing_fluttershy_by_moongazeponies-d3fiy2b.png

Trixie: http://img06.deviantart.net/09e3/i/2011/194/9/e/trixie_by_shelmo69-d3nxapz.png