Introduction To Arp - 3Com 8807 Configuration Manual

ARP C
ONFIGURATION
55

Introduction to ARP

Address resolution protocol (ARP) is used to resolve an IP address into a MAC
address.
Necessity of ARP
An IP address cannot be directly used for communication between network
devices because network devices can identify only MAC addresses. An IP address is
only an address of a host in the network layer. To send datagrams through the
network layer to the destination host, the MAC address of the host is required. So
the IP address must be resolved into a MAC address.
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 mappings
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 needs
to transmit messages to Host B.
ARP implementation procedure is as follows: 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 puts the IP packet into
the send queue, 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 corresponding MAC address
of Host B and adds them to its own ARP mapping table. Then Host A sends Host B
all the packets standing in the queue.