Introduction To Arp - 3Com 7757 Configuration Manual

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Introduction to ARP

Necessity of ARP
ARP Packet Format
ARP C
ONFIGURATION
Address Resolution Protocol (ARP) is used to map network layer protocol addresses
(IP addresses) to corresponding data link layer hardware addresses (MAC
addresses).
Network devices can directly identify Layer 2 MAC addresses instead of Layer 3 IP
addresses. For a Layer 3 packet to be received by its destination host, it must carry
the MAC address of the destination host. So, before sending a packet, the source
device must map the destination IP address to the MAC address of the destination
device.
There are two types of ARP packets: ARP request and ARP reply. Figure 146
illustrates the structure of the two types of ARP packets. In this figure:
All fields except for the target hardware address field are used in an ARP
■
request. The target hardware address is just what the sender wants to obtain.
All fields are used in an ARP reply.
■
Figure 146 ARP packet format
Hardware type (16 bits) Hardware type (16 bits) Hardware type (16 bits)
Protocol type (16 bits) Protocol type (16 bits)
Length of hardware address Length of protocol address Length of hardware address Length of protocol address
Operator (16 bits) Operator (16 bits)
Hardware address of the sender Hardware address of the sender
IP address of the sender IP address of the sender
Hardware address of the receiver Hardware address of the receiver
IP address of the receiver IP address of the receiver
Table 444 describes the fields of an ARP packet.
Table 444 Field descriptions of an ARP packet
Field
Hardware Type
Description
Type of the hardware interface. See Table 445 for the
valid values of this field.