Elements of IP Routing 4-7
Address Resolution
Protocol (ARP)
ARP is a low-level protocol used to locate the MAC address
corresponding 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.
When the host or router knows the IP address of the next hop toward a
packet’s 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 participating in IP
routing maintains an ARP cache. See Figure 4-6.
Figure 4-6 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 both the hardware (MAC addresses) and the
protocol (IP addresses). See Figure 4-7.
Figure 4-7 Example of an ARP Request Packet
ARP cache
158.101.1.1
158.101.2.1
IP address MAC address
00308e3d0042
0080232b00ab
158.101.2.15
158.101.2.1
Source hardware address
?
ARP request packet
Source protocol address
Target protocol address
Target hardware address
00802322b00ad
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