ZyXEL Communications GS2200-24 Series, User Manual

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GS2200-24 User’s Guide
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H A P T E R
38
ARP Table
38.1 Overview
This chapter introduces 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.
38.2 What You Can Do
Use the ARP Table screen ( Section 38.3 on page 320 ) to view IP-to-MAC address
mapping(s).
38.2.1 How ARP Works
When an incoming packet destined for a host device on a local area network
arrives at the Switch, the Switch'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.