ZyXEL Communications XS3800-28, User Manual, Page: 461 / 789

Chapter 63 ARP Setup
XS3800-28 User’s Guide
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ARP Setup
63.1 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.
63.1.1 What You Can Do
• Use the ARP Learning screen ( Section 63.2 on page 463 ) to configure ARP learning mode on a per-
port basis.
• Use the Static ARP screen ( Section 63.3 on page 465 ) to create static ARP entries that will display in the
MONITOR > ARP Table screen and will not age out.
63.1.2 What You Need to Know
Read on for concepts on ARP that can help you configure the screen in this chapter.
63.1.2.1 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.
63.1.2.2 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