3Com Switch 8800 Configuration Guide Chapter 12 ARP Configuration
12-1
Chapter 12 ARP Configuration 12.1 Introduction to ARP
Address Resolution Protocol (ARP) is used to resolve an IP address into a MAC
address.
I. Necessity of ARP
An IP address cannot be directly used for communication between network devices
because network devices can only identify MAC addresses. An IP address is only an
address of a host in the network layer. To send the data packet s transmitted through the
network layer to the destination host, MAC address of the host is required. So the IP
address must be resolved into a MAC address.
II. ARP implementation procedure
When two hosts on the Ethernet need to communicate with each other, they must know
the MAC addresses of each other. Every host maintains the IP-MAC address
translation table, which is known as the ARP mapping table. A series of maps between
IP addresses and MAC addresses of other hosts which recently communicate with the
local host are stored in the ARP mapping table. When a dynamic ARP mapping entry is
not in use for a specified period of time, the host removes it from the ARP mapping
table so as to save the memory space and shorten the interval for the switch to search
ARP mapping table.
Suppose there are two hosts on the same network segment: Host A and Host B. The IP
address of Host A is IP_A and the IP address of Host B is IP_B. Host A will transmit
messages to Host B. Host A checks its own ARP mapping table first to know whether
there are corresponding ARP entries of IP_B in the table. If the corresponding MAC
address is found, Host A uses the MAC address in the ARP mapping table to
encapsulate the IP packet in frame and sends it to Host B. If the corresponding MAC
address is not found, Host A stores the IP packet in the queue waiting for transmission,
create an ARP request packet and broadcast it throughout the Ethernet. The ARP
request packet contains the IP address of Host B and IP address and MAC address of
Host A. Since the ARP request packet is broadcasted, all hosts on the network
segment can receive the request. However, only the requested host (namely, Host B)
needs to process the request. Host B first stores the IP address and the MAC address
of the request sender (Host A) in the ARP request packet in its own ARP mapping table.
Then, Host B generates an ARP reply packet by adding its own MAC address into the
packet, and then send it to Host A. The reply packet is directly sent to Host A in stead of
being broadcasted. Receiving the reply packet, Host A extracts the IP address and the