Arp Table; Chapter 25 Arp Table; Overview; How Arp Works - ZyXEL Communications VMG1312-B Series User Manual

25.1 Overview

Address Resolution Protocol (ARP) is a protocol for mapping an Internet Protocol address (IP
address) to a physical machine address, also known as a Media Access Control or MAC address, on
the local area network.
An IP (version 4) address is 32 bits long. In an Ethernet LAN, MAC addresses are 48 bits long. The
ARP Table maintains an association between each MAC address and its corresponding IP address.

25.1.1 How ARP Works

When an incoming packet destined for a host device on a local area network arrives at the device,
the device's ARP program looks in the ARP Table and, if it finds the address, sends it to the device.
If no entry is found for the IP address, ARP broadcasts the request to all the devices on the LAN.
The device fills in its own MAC and IP address in the sender address fields, and puts the known IP
address of the target in the target IP address field. In addition, the device puts all ones in the target
MAC field (FF.FF.FF.FF.FF.FF is the Ethernet broadcast address). The replying device (which is either
the IP address of the device being sought or the router that knows the way) replaces the broadcast
address with the target's MAC address, swaps the sender and target pairs, and unicasts the answer
directly back to the requesting machine. ARP updates the ARP Table for future reference and then
sends the packet to the MAC address that replied.
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ARP Table