Arp Setup - ZyXEL Communications GS2210-8 User Manual

41.1 ARP 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 t able m aint ains an associat ion bet ween each MAC address and it s corresponding I P address.
41.1.1 What You Can Do
Use t he ARP Le a r n in g screen (
per- port basis.
41.1.2 What You Need to Know
Read on for concept s on ARP t hat can help you configure t he screen in t his chapt er.
41.1.2.1 How ARP Works
When an incom ing packet dest ined for a host device on a local area net work arrives at t he Swit ch,
t he Swit ch looks in t he ARP Table and if it finds t he address, it 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 Swit ch 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 Swit ch 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.
41.1.2.2 ARP Learning Mode
The Swit ch support s t hree ARP learning m odes: ARP- Reply, Grat uit ous- ARP , and ARP- Request .
ARP-Reply
The Swit ch in ARP- Reply learning m ode updat es t he ARP t able only wit h t he ARP replies t o t he ARP
request s sent by t he Swit ch. This can help prevent ARP spoofing.
I n t he following exam ple, t he Swit ch does not have I P address and MAC address m apping
inform at ion for host s A and B in it s ARP t able, and host A want s t o ping host B. Host A sends an
C
HAPTER
Sect ion 41.2.1 on page 351) t o configure ARP learning m ode on a
GS2210 Series User's Guide
349
4 1

ARP Setup