The Address Resolution Protocol
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IP routing logic requires that hosts and routers encapsulate IP packets inside data link layer
frames. In fact, Figure 4-11 shows how every router de-encapsulates each IP packet and
encapsulates the IP packet inside a new data-link frame.
On Ethernet LANs, whenever a host or router needs to encapsulate an IP packet in a new
Ethernet frame, the host or router knows all the important facts to build that header—
except for the destination MAC address. The host knows the IP address of the next device,
either another host IP address or the default router IP address. A router knows the IP route
used for forwarding the IP packet, which lists the next router’s IP address. However, the
hosts and routers do not know those neighboring devices’ MAC addresses beforehand.
TCP/IP defines the Address Resolution Protocol (ARP) as the method by which any host
or router on a LAN can dynamically learn the MAC address of another IP host or router
on the same LAN. ARP defines a protocol that includes the ARP Request, which is a message
that asks the simple request “if this is your IP address, please reply with your MAC
address.” ARP also defines the ARP Reply message, which indeed lists both the original IP
address and the matching MAC address.
Figure 4-14 shows an example that uses the same router and host from the bottom part of
the earlier Figure 4-11. The figure shows the ARP Request on the left as a LAN broadcast,
so all hosts receive the frame. On the right, at Step 2, host PC2 sends back an ARP Reply,
identifying PC2’s MAC address. The text beside each message shows the contents inside the
ARP message itself, which lets PC2 learn R3’s IP address and matching MAC address, and
R3 learn PC2’s IP address and matching MAC address.
Figure 4-14 Sample ARP Process
Note that hosts remember the ARP results, keeping the information in their ARP cache or
ARP table. A host or router only needs to use ARP occasionally, to build the ARP cache
the first time. Each time a host or router needs to send a packet encapsulated in an Ethernet
frame, it first checks its ARP cache for the correct IP address and matching MAC address.
Hosts and routers will let ARP cache entries time out to clean up the table, so occasional
ARP Requests can be seen.
NOTE You can see the contents of the ARP cache on most PC operating systems by using
the arp -a command from a command prompt.