3com DUA1750-2BAA01 Manual De Usuario

3Com Switch 8800 Configuration Guide 

Chapter 12  ARP Configuration

 

12-1 

Address Resolution Protocol (ARP) is used to resolve an IP address into a MAC 

address. 

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 packets 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. 

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