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137

MSAP2000 AAMS User’s Guide

C HAPTER 23

ARP Table

This chapter introduces the ARP table.

23.1 Introduction to ARP Table

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.

23.1.1 How ARP Works

When an incoming packet destined for a host device on a local area network

arrives at the switch, the switch'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 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.

23.2 Viewing the ARP Table

Click Management in the navigation panel and then ARP Table to open the

following screen.

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ZyXEL MSAP2000 Manuel d'utilisateur Page 137