ZyXEL Communications ES-2048, User Manual

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ES-2048 User’s Guide
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H A P T E R
32
ARP Table
This chapter introduces ARP Table.
32.1 ARP Table 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 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.
32.2 Viewing the ARP Table
Click Management > ARP Table in the navigation panel to open the following screen. Use
the ARP table to view IP-to-MAC address mapping(s).