Arp Setup; Chapter 40 Arp Setup; Arp Overview - ZyXEL Communications MES3500-24S User Manual

40.1 ARP 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.
40.1.0.1 How ARP Works
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.
40.1.0.2 ARP Learning Mode
The Switch supports three ARP learning modes: ARP-Reply, Gratuitous-ARP, and ARP-Request.
ARP-Reply
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 A and B in its ARP table, and host A wants to ping host B. Host A sends an ARP request to the
Switch and then sends an ICMP request after getting the ARP reply from the Switch. The Switch finds no
matched entry for host B in the ARP table and broadcasts the ARP request to all the devices on the LAN.
When the Switch receives the ARP reply from host B, it updates its ARP table and also forwards host A's
ICMP request to host B. After the Switch gets the ICMP reply from host B, it sends out an ARP request to
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ARP Setup

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