3Com Switch 7754, Configuration Manual, Page: 571 / 940

54
ARP C
ONFIGURATION
Introduction to ARP
Address Resolution Protocol (ARP) is used to map network layer protocol addresses
(IP addresses) to corresponding data link layer hardware addresses (MAC
addresses).
Necessity of ARP 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.
ARP Packet Format 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
Table 444 describes the fields of an ARP packet.
Table 444 Field descriptions of an ARP packet
Field Description
Hardware Type Type of the hardware interface. See Table 445 for the
valid values of this field.
Hardware type (16 bits)
Protocol type (16 bits)
Length of hardware address Length of protocol address
Operator (16 bits)
Hardware address of the sender
IP address of the sender
Hardware address of the receiver
IP address of the receiver
Hardware type (16 bits) Hardware type (16 bits)
Protocol type (16 bits)
Length of hardware address Length of protocol address
Operator (16 bits)
Hardware address of the sender
IP address of the sender
Hardware address of the receiver
IP address of the receiver