ARP – Address Resolution Protocol
This protocol is a process which maps out the IP address of every machine that is connected to a local area network. This is basically a program that would contain the location of every computer in a building that is connected to a single router.
This can be compared to Google maps. Google maps has the address of almost every home, business, etc. in the USA.
If you are searching for a business in another state you can type in the address of that business and Google maps would show you exactly where it is in the matter of a few seconds.
This is what ARP does for the gateway. When a message or packet comes to the gateway the gateway asks the ARP to do a search of the network and show it how to get to the address that is on the packet.
The ARP then responds with a map of all of the computers on the network and shows the gateway how to send the packet along the network to reach its final destination.