Address Resolution Protocol (ARP) is used when a device knows the IP address of a remote device, but not its MAC address.
If the Layer 3 address (the IP address) of the remote device is known, why does the Layer 2 address need to be found? When the data is sent, the destination MAC address must be sent with it.
As network devices learn the MAC addresses of other devices on the network, they build ARP caches. This local cache is checked for the proper MAC address, and if the MAC address is not found here, ARP will send out a broadcast containing the known IP address. The remote device with the matching IP address will respond with its MAC address. All other devices will ignore the ARP request.
Reverse Address Resolution Protocol (RARP) is used in the opposite situation; the MAC address is known and the IP address is not. Used when a workstation is diskless (that is, the workstation does not have its own hard drive), because the diskless workstation has no way to know its own IP address. It will know its own hardware address, though.
The diskless workstation will send out a packet with its MAC address and a request for its own IP address. A device specifically configured to respond to this request, the RARP server, will send a packet back to the diskless workstation containing the desire IP address.
If the Layer 3 address (the IP address) of the remote device is known, why does the Layer 2 address need to be found? When the data is sent, the destination MAC address must be sent with it.
As network devices learn the MAC addresses of other devices on the network, they build ARP caches. This local cache is checked for the proper MAC address, and if the MAC address is not found here, ARP will send out a broadcast containing the known IP address. The remote device with the matching IP address will respond with its MAC address. All other devices will ignore the ARP request.
Reverse Address Resolution Protocol (RARP) is used in the opposite situation; the MAC address is known and the IP address is not. Used when a workstation is diskless (that is, the workstation does not have its own hard drive), because the diskless workstation has no way to know its own IP address. It will know its own hardware address, though.
The diskless workstation will send out a packet with its MAC address and a request for its own IP address. A device specifically configured to respond to this request, the RARP server, will send a packet back to the diskless workstation containing the desire IP address.