This is the text from the movie entitled "The Address Resolution Protocol", published at www.AskMisterWizard.com.

Ethernet Delivers the Internet Part 2: The Address Resolution Protocol ( ARP )

Computers with Internet access generally use at least - TWO - different addressing systems at the same time. Usually this includes:

1 of 2: an Ethernet Address (functioning at layer 2 of the  OSI Reference Model), and

2 of 2: an  IP Address (functioning at layer 3 of the OSI Reference Model).

Ethernet addresses are assigned when an Ethernet interface is manufactured, before any relationship is established with you or your network. Accordingly, the various Ethernet addresses found within different computers in your home or small office may appear to have no relationship with one another. You can learn more about Ethernet and Ethernet addressing from the - "Ethernet Series" - of movies at AskMisterWizard.com, where you will find at least six short movies are always available for downloading.

IP addresses, on the other hand, are assigned in a coordinated system as your IP network grows, ensuring that adjacent computers within your local network have similar addresses.

Internet Addresses - are IP addresses that are coordinated globally, and the structure of an Internet address helps computers and routers to locate one another anywhere in the world. In this movie, we will use the vocabulary of the well-known " OSI Reference Model " to describe Ethernet addresses and IP addresses, and you will need to understand the basics of both. You can learn more about IP and Internet addresses from the extensive - "Internet Series" - of movies at AskMisterWizard.com.

You can learn more about the OSI reference model (and the resulting - "layers" - and vocabulary) from the AskMisterWizard.com movie entitled - "The 7-Layer Vocabulary of Modern Internetworking".

When your computer wants to send an IP packet to another, it must examine the destination IP address and make a decision as to whether the recipient can be found on your - LOCAL - network. This is done by comparing the destination IP address with the IP addresses of any and all network interfaces inside the originating computer.

If the addresses are similar, then the associated - "Netmask" - value is used to determine the size of the local network(s) and finalize the - "Local or Remote" - decision.

If the destination is - outside - of your local network, then the packet can be given to the router that serves as your network's - Default Gateway - and the routers within the worldwide Internet assume responsibility for delivery.

To begin this process, your local computer encapsulates the outgoing IP packet inside an Ethernet frame, retrieves necessary Ethernet addressing information from your operating system's carefully managed, pre-existing notes, and uses it's Ethernet Interface to send the IP packet to the Ethernet address of your local router, which routes it to the Internet through your Internet Service Provider.

On the other hand, if the destination is - inside - your local network, then the outgoing IP packet is encapsulated inside an Ethernet packet that is addressed directly to the Ethernet interface of the intended recipient.

(To understand the example shown here, it's important to remember that popular, low-cost NAT routers include an Ethernet Hub or Switch. In this case, that Ethernet logic - packaged inside the NAT router - relays the Ethernet frame directly to the Ethernet destination without using layer-3 routing at all).

In either case, just knowing the destination - IP - address is not enough for your operating system. Your computer will need to learn the local - Ethernet - Address to which it should forward the Ethernet frame for the short hop to the next step.

Back in the early days of Internet design, the early Internet wizards realized that they would need some simple mechanisms to translate back and forth between Ethernet addresses and IP addresses. If you've seen the AskMisterWizard.com movie entitled - "Configuring Your Internet Connection Part 1: The Dynamic Host Control Protocol" - then you already know how an Ethernet-equipped computer can learn it's IP address.

In - this - movie we explore the opposite translation: How a computer in possession of the - IP - address of one of it's local neighbors can learn the corresponding - Ethernet - address. Faced with this problem, the early Internet designers came up with a clever mechanism that they named the - "Address Resolution Protocol", or - " ARP " .

The movies in our - "Ethernet Series" - explained that Ethernet uses a - "broadcast" - mechanism, and that modern Ethernet Switches try to minimize broadcast traffic by limiting transmissions over unrelated interfaces. However, even in modern Ethernets in which old-fashioned - "Hubs" - have been replaced with Ethernet switches, some tasks work better if Ethernet frames are broadcast to every Ethernet interface that can be reached on the local network. Address Resolution Protocol takes advantage of this "broadcast" facility, and all ARP messages are broadcast to all local Ethernet interfaces. The protocol works so well that it has become completely automatic and virtually invisible to ordinary users. You should never need to configure, install, or remove any ARP components of your modern operating system. We are including this movie on ARP only to help you understand how your network works. (This information may also be helpful to you if the ARP logic of your local network is ever attacked by a hostile insider, who may be able to use it to subvert your Ethernet switches).

When your computer needs to know the Ethernet address of another computer whose IP address indicates that it should reside within your local network, it broadcasts an Ethernet message that looks like this:

Hey! Everybody Listen Up! According to my analysis, the IP address 192.168.0.2 should belong to somebody on this Ethernet LAN, and I have an IP message for that destination. If - YOU - are the station that has been assigned that IP address, please respond back to me on - my - Ethernet address so - I - can collect - your - Ethernet address.

If the destination address is incorrect, unassigned, or if the destination computer is switched off or disconnected, then no response message will be received, and your computer will abandon the attempt to send the message.

Under normal circumstances on a healthy network, within a few milliseconds, an answering message will be received like this:

" - I - am using IP address 192.168.0.2 and my Ethernet Address is 31, 22, 3D, 01, 01, A2."

That message provides all of the information your computer needs, and it will construct an Ethernet frame directed at Ethernet Address 31, 22, 3D, 01, 01, A2, containing the IP packet needing transmission. Your local computer's operating system will automatically store a note within a special, temporary memory area known as the - "ARP cache", - recording the received IP and Ethernet address relationship for future reference. Typically, the ARP cache information is held for only a few minutes (so it is handy for long enough to complete current tasks) before it is automatically cleared out.

Conclusions

Modern networks include an automated mechanism by which computers can learn the Ethernet address associated with an IP address on the local network. The mechanism relies on a simple Ethernet broadcast and response dialog, and is completely automated. Unless the ARP system of your local network is explicitly attacked by a hostile insider, you should never need to reinstall or configure it.

This is the text from the movie entitled "The Address Resolution Protocol", published at www.AskMisterWizard.com. (The text also includes a few small, static illustrations from the movie).