ZyXEL Communications ONU-2024 Series, User Manual

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ONU User’s Guide
231
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H A P T E R
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ARP Table
32.1 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.
32.1.1 What You Can Do in the ARP Table Screen
The ARP Table screen allows you to view IP-to-MAC address mappings ( Section 32.2 on
page 232 ).
32.1.2 What You Need to Know About the ARP Table Screen
The following terms and concepts may help as you read through this chapter.
How ARP Works
When an incoming packet destined for a host device on a local area network arrives at the
ONU, the ONU's ARP program looks in the ARP Table and, if it finds the address, 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 ONU 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 ONU 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.