E-Lab 4.1.6a - Address Resolution Protocol
Address Resolution Protocol (ARP) is a protocol for mapping an IP address to a physical device address in a local area network.
When an incoming packet destined for a host machine on a particular local area network arrives at a gateway, the gateway asks the ARP program to find a physical host or MAC address that matches the IP address. 
The ARP program looks in the ARP cache and, if it finds the address, provides it so that the packet can be converted to the right packet length and format and sent to the machine.
If no entry is found for the IP address, ARP broadcasts a request (ARP Request) packet in a special format to all the machines on the LAN to see if one machine knows that it has that IP address associated with it.
A machine that recognizes the IP address as its own returns a reply (ARP Reply).
ARP updates the ARP cache for future reference and then sends the packet to the MAC address that replied.
Assume all IP addresses, host names, passwords and static routing has been set.
Step 1
Enter into privileged EXEC mode.
Step 2.1
A serial interface on a router does not have a MAC address associated with it.
However, Ethernet interfaces do.
Use the show interfaces command to display the MAC address of interface Ethernet 0.
Step 2.2
Now display the same information for the second ethernet interface.
Step 2.3
Notice here each interface has two MAC addresses.
The first MAC address displayed is the configurable MAC address and the second address is the burned in MAC address.
The default timeout value for ARP is set to four hours.
The amount of time before ARP entries are aged and removed from the ARP table can be changed on interface basis by using the arp timeout command.
Using the command show arp, view the ARP table.
Step 3.1
Notice that Hanoi only contains two entries, one for each of its Ethernet interfaces.
Type the debug aro command to view the ARP message being sent.
Step 3.2
Run a ping from Workstation A to Hanoi (192.168.1.1).
Step 3.3
View the ARP table on Hanoi to view the new entries.
Step 3.4
To communicate between Hanoi and Workstation A, both devices require the MAC address of the other.
In these examples the ping command is used to initiate ARP requests and replies.
Keep in mind that any communication between applications of different nodes require the source node to have knowledge of the MAC address of the destination node or the MAC address of the gateway before any successful communication can occur.
Type arp -a to view the workstation ARP table.