3Com CoreBuilder 3500, Руководство по внедрению, Страница: 286 / 592

286 C
HAPTER
11: I
NTERNET
P
ROTOCOL
(IP)
Address Resolution
Protocol (ARP)
ARP is a low-level protocol that locates the MAC address that
corresponds to a given IP address. This protocol allows a host or router to
use IP addresses to make routing decisions while it uses MAC addresses
to forward packets from one hop to the next.
You do not need to implement ARP — your system has ARP capability
built in, but you can manipulate and display the contents of the ARP
cache.
When the host or router knows the IP address of the
next
hop towards
the packet destination, the host or router translates that IP address into a
MAC address before sending the packet. To perform this translation, the
host or router first searches its
ARP cache , which is a table of IP addresses
with their corresponding MAC addresses. Each device that participates in
IP routing maintains an ARP cache. See Figure 53.
Figure 53 Example of an ARP Cache
If the IP address does not have a corresponding MAC address, the host or
router broadcasts an
ARP request
packet to all the devices on the
network. The ARP request contains information about the target and
source addresses for the protocol (IP addresses). See Figure 54.
ARP cache
158.101.1.1
158.101.2.1
IP address MAC address
00308e3d0042
0080232b00ab