When two systems located on the same LAN want to communicate, they do not transfer data to and from systems based on IP address but rather than the MAC address. Unlike an IP address that can be assigned so duplicates exist, no two network interfaces "should" have the same MAC address ensuring each system is uniquely represented.
IP comes into play when systems located on different networks need to communicate, allow that communication to be routed to the remote destination through a series of routers based on destination IP.
I would review
http://en.wikipedia.org/wiki/Address_Resolution_Protocol for additional information.
Hope this helps...
Mike
