Arp Table; Chapter 22 Arp Table; Overview; How Arp Works - ZyXEL Communications VMG1312-B Series User Manual

22.1 Overview

Address Resolut ion Prot ocol ( ARP) is a prot ocol for m apping an I nt ernet Prot ocol address ( I P
address) t o a physical m achine address, also known as a Media Access Cont rol or MAC address, on
t he local area net work.
An I P ( version 4) address is 32 bit s long. I n an Et hernet LAN, MAC addresses are 48 bit s long. The
ARP Table m aint ains an associat ion bet ween each MAC address and it s corresponding I P address.

22.1.1 How ARP Works

When an incom ing packet dest ined for a host device on a local area net work arrives at t he device,
t he device's ARP program looks in t he ARP Table and, if it finds t he address, sends it t o t he device.
I f no ent ry is found for t he I P address, ARP broadcast s t he request t o all t he devices on t he LAN.
The device fills in it s own MAC and I P address in t he sender address fields, and put s t he known I P
address of t he t arget in t he t arget I P address field. I n addit ion, t he device put s all ones in t he t arget
MAC field ( FF.FF.FF.FF.FF.FF is t he Et hernet broadcast address) . The replying device ( which is eit her
t he I P address of t he device being sought or t he rout er t hat knows t he way) replaces t he broadcast
address wit h t he t arget 's MAC address, swaps t he sender and t arget pairs, and unicast s t he answer
direct ly back t o t he request ing m achine. ARP updat es t he ARP Table for fut ure reference and t hen
sends t he packet t o t he MAC address t hat replied.

22.2 ARP Table Screen

Use t he ARP t able t o view I P- t o- MAC address m apping( s) . To open t his screen, click Syst e m
M onit or > ARP Ta ble .
Figure 122 Syst em Monit or > ARP Table
The following t able describes t he labels in t his screen.
Table 94 Syst em Monit or > ARP Table
LABEL
#
I P Address
VMG1312-B Series User's Guide
DESCRIPTION
This is t he ARP t able ent ry num ber.
This is t he learned I P address of a device connect ed t o a port .
C
HAPTER
2 2

ARP Table

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