ZyXEL Communications AAM1212-51, User Manual

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AAM1212-51/53 Series User’s Guide
Chapter 40 ARP Table 249
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H A P T E R
40
ARP Table
This chapter describes the ARP Table.
40.1 Introduction to ARP Table
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.
40.1.1 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.
40.2 ARP Table Screen
The ARP table can hold up to 500 entries.
To open this screen, click Management, ARP Table .