1-11 ARP ConfigurationWhen configuring ARP, go to these sections for information you are interested in:z ARP Overviewz Configuring ARPz Configuring Gratuitous ARPz Displaying and Maintaining ARPARP OverviewARP FunctionThe Address Resolution Protocol (ARP) is used to resolve an IP address into an Ethernet MAC address(or physical address).In a LAN, when a device is to send data to another device, the sending device must know the networklayer address (that is, the IP address) of the destination device. Because IP datagrams must beencapsulated within Ethernet frames before they can be transmitted over physical networks, thesending device also needs to know the physical address of the destination device. Therefore, amapping between the IP address and the physical address is needed. ARP is the protocol to implementthe mapping function.ARP Message FormatARP messages are classified into ARP requests and ARP replies. Figure 1-1 shows the format of theARP request/reply.Figure 1-1 ARP message formatThe following explains the fields in Figure 1-1.z Hardware type: This field specifies the hardware address type. The value “1” represents Ethernet.z Protocol type: This field specifies the type of the protocol address to be mapped. The hexadecimalvalue “0x0800” represents IP.z Hardware address length and protocol address length: They respectively specify the length of ahardware address and a protocol address, in bytes. For an Ethernet address, the value of thehardware address length field is "6”. For an IP(v4) address, the value of the protocol address lengthfield is “4”.
