Ramblings & ephemera

How ARP works

From Chris Sanders’ “Packet School 201 – Part 1 (ARP)” (Completely Full of I.T.: 23 December 2007):

The basic idea behind ARP is for a machine to broadcast its IP address and MAC address to all of the clients in its broadcast domain in order to find out the IP address associated with a particular MAC address. Basically put, it looks like this:

Computer A – “Hey everybody, my IP address is XX.XX.XX.XX, and my MAC address is XX:XX:XX:XX:XX:XX. I need to send something to whoever has the IP address XX.XX.XX.XX, but I don’t know what their hardware address is. Will whoever has this IP address please respond back with their MAC address?

All of the other computers that receive the broadcast will simply ignore it, however, the one who does have the requested IP address will send its MAC address to Computer A. With this information in hand, the exchange of data can being.

Computer B – “Hey Computer A. I am who you are looking for with the IP address of XX.XX.XX.XX. My MAC address is XX:XX:XX:XX:XX:XX.

One of the best ways I’ve seen this concept described is through the limousine driver analogy. If you have ever flown, then chances are when you get off of a plane, you have seen a limo driver standing with a sign bearing someone’s last name. Here, the driver knows the name of the person he is picking up, but doesn’t know what they look like. The driver holds up the sign so that everyone can see it. All of the people getting off of the plane see the sign, and if it isn’t them, they simply ignore it. The person whose name is on the card however, sees it, approaches the driver, and identifies himself.

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