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ARP Explained - Address Resolution Protocol

AARP stands for address resolution protocol now this is a protocol that is used to resolve IP addresses to MAC addresses the MAC address is a physical address of a device it's a globally unique number that is assigned to every network interface card whenever a device needs to communicate with another device on a local area network it needs the MAC address for that device and devices use ARP to acquire the MAC address for that device so as an example let's say that computer a wants to communicate with computer B now computer a already knows the IP address for computer B but in order to communicate with computer B it still needs its MAC address now an IP address is used to locate a device on a network and the MAC address is what identifies the actual device so in order to find the MAC address computer a will first look at its internal list called an ARP cache to see if computer B's IP address already has a matching MAC address in fact we can check this ourselves at a Windows command prompt by using the ARP utility and by typing ARP space minus a and as you can see in the output it has no entries at all the ARP cache is empty so now computer a will send out a broadcast message out on the network asking every device which computer has the specific IP address and will ask for their MAC address then the computer that has the matching IP address will then respond back and tell computer a its MAC address then once it receives the MAC address the communication can now take place between the two now once computer a has the MAC address it'll store this information in the ARP cache so now let's do the same commands as before and now you can see that the IP address and matching MAC address have been added to the ARP cache the ARP cache is used to make a network more efficient it stores IP address 2 MAC address associations so that the next time it needs to communicate with a device on the network it doesn't have to broadcast a message out on the entire network it can just look in the ARP cache and there are also two different types of AARP entries dynamic and Static a dynamic entry is created automatically when a device sends out a broadcast message out on the network requesting a MAC address just like in the example we just did dynamic entries are not permanent they are flushed out periodically so that the cache doesn't get filled up with too many entries that are not being used and a static entry is where someone manually enters an IP address 2 MAC address association using the ARP command-line utility so for example let's create a static ARP entry so at a command prompt you would type ARP space minus s and then the IP address followed by the MAC address so now if we look at the ARP cache again we would see our static ARP entry network administrators would use static entries to reduce any unnecessary ARP broadcast traffic on a network for example static entries are ideal if you know that two devices are constantly going to be communicating with each other
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