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What is an ARP packet?
ARP stands for Address Resolution Protocol.
ARP is a method by which IP Addresses are mapped to physical hardware addresses before packets are sent across the wire to another computer.
If you'll remember how a computer actually sends data from one machine to the next, it has to go down the TCP/IP Stack.
Application Layer sends to the Transport Layer (TCP) Transport Layer sends to the Network Layer (IP) Network Layer sends to the Data Link Layer (for most networks, Ethernet)
ARP is essentially a method where by knowing the IP Layer, discovering what the frame layer should look like is discovered.
If you'll remember, most switches that you plug a computer into are not aware of the IP layer, they work at the Network Layer.
ARP works by sending out a broadcast request to the network, asking for the MAC Address of a specific IP.
When computers respond with the corresponding MAC Address, this address is cached in a ARP Table on the local computer.
In most systems the arp command will show you the listing of the local arp table and it's cached address list.
So even though "ARP packet" may not be the exact correct term, because you could call it ARP frame request, the basic idea is that by sending out an ARP Request, the local computer is able to identify the MAC Address of another computer when it only knows it's IP Address.