Arp Setup; C Ha Pte R 38 Arp Se Tup; Arp Overview; What You Can Do - ZyXEL Communications GS1920v2 Series User Manual

38.1 ARP O ve rvie w
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.
38.1.1 Wha t Yo u C a n Do
Use the
ARP L e a rning
basis.
38.1.2 Wha t Yo u Ne e d to Kno w
Read on for concepts on ARP that can help you configure the screen in this chapter.
38.1.2.1 Ho w ARP Wo rks
When an incoming packet destined for a host device on a local area network arrives at the Switch, the
Switch looks in the ARP Table and if it finds the address, it 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
Switch 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 Switch 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.
38.1.2.2 ARP L e a rning Mo de
The Switch supports three ARP learning modes: ARP-Reply, Gratuitous-ARP, and ARP-Request.
ARP- Re ply
The Switch in ARP-Reply learning mode updates the ARP table only with the ARP replies to the ARP
requests sent by the Switch. This can help prevent ARP spoofing.
In the following example, the Switch does not have IP address and MAC address mapping information
for hosts and
A
screen (Section 38.2.1 on page
in its ARP table, and host
B
GS1920v2 Series User's Guide
C
HA PTER
ARP Se tup
337) to configure ARP learning mode on a per-port
wants to ping host . Host
A B
335
3 8
sends an ARP request to the
A