What is an ARP?

Delana Hallstedt. My original plan for this blog was to try to unravel the alphabet soup approach in all things technology-related – Why so many acronyms?! After doing some research on the origins and meanings of acronyms (retronyms as well as neologism and alphabetism), I was able to change my mind. I quickly resigned myself to that decision and settled on one acronym.
Okay, that was two. In my defense, that was two. I plan to blog about the first one. I’ll just have to wait and see if my flagrant use for random letters isn’t squashed by an editors 🙂
Address Resolution Protocol, also known as ARP, is used for mapping an IP address to a physical address. ARP is a broadcast protocol and is located at the border of the Internet layer (or Network Access layer). ARP can dynamically map an address to a hardware address such as an Ethernet Media Access Control address (MAC) address.
ARP is used by the Internet layer to determine the hardware address associated to a particular IP address. ARP will determine the hardware address associated with a Protocol Data Unit (PDU)’s destination IP address if it is within the same network as an interface on the host. ARP will not determine the hardware address associated to the destination IP address if the PDU’s destination address is in a network to the host. Once the mapping is determined, the Internet layer can instruct Network Access to forward the PDU to that hardware address.
An example of this is an ARP request that uses an Ethernet network destination MAC address of FF.FF:FF.FF:FF.FF. It is received by all hosts on a local LAN segment. The request simply states “Who is IP Address x.x.x.x?” Tell IP address y.y.y.y. The ARP request uses a hardware broadcast adress. Each receiving host forwards the request through its Network Access layer to the ARP process. The ARP request will recognize the host’s IP address and send an ARP reply to the specified IP address. The querying host will save the IP address-to MAC address mapping in its ARP database for future reference.
Reverse ARP is a similar protocol. It maps a hardware address to an IP address. If a host has not been assigned a static IP address, RARP can be used to dynamically find an IP address. RARP has been largely replaced by Dynamic Host Configuration Protocol, (DHCP), because it is more robust. It is also easier to manage.
The arp -a command allows you to view the ARP table of a Microsoft Windows host. This command output shows some sample mappings that you might find if you issue the arp -a command to display the contents a host’s cache ARP.
C:>arp Interface: — 0x2 Internet address Physical Address Type–0a dynamic 00-11–43-ed-20–f0 dynamic 0.0-0c-29–71-7c–ae dynamic
You can also issue the show IP arp command to display ARP tables on multiple Cisco devices.
FTR (for record), IP and DHCP technically are initialisms, but not acronyms. Initialisms can be spoken one letter at time. ARP and MAC are true acronyms, however, because they are pronounced as a whole, not the individual letters. It turns out that acronym usage has been around since the Roman Empire. The IT industry made it even more popular in the 20th Century. I didn’t want to leave you hanging but I didn’t spend a thousand words trying tell you how the IT industry powers like to name things that are very descriptive of the actual function of the thing.