| Lesson 10 | ARP and RARP |
| Objective | Address Resolution Protocol and Reverse Address Resolution Protocol |
What is the purpose of Address Resolution Protocol and Reverse Address Resolution Protocol?
ARP is used to mediate between ethernet (or other broadcast link-level protocols) and the network layer, or IP protocols. Put more simply, ARP converts IP addresses to ethernet addresses.
Why ARP is necessary
ARP is used to mediate between ethernet (or other broadcast link-level protocols) and the network layer, or IP protocols. Put more simply, ARP converts IP addresses to ethernet addresses.
ARP is necessary because the underlying ethernet hardware communicates using ethernet addresses, not IP addresses. Suppose that one machine,
with IP address 2 on an ethernet network, wants to speak to another machine on the same network with IP address 8. The two machines use ARP to
conduct the following dialogue: ARP Request—Machine 1 (IP=2) broadcasts to all machines on the network: Who has IP address 8? ARP
Reply—Machine 2 (IP=8) replies: I do! Machine 2's reply contains its ethernet address, so now Machine 1 knows it. Machine 1 stashes that
address temporarily in a kernel memory area called the ARP cache. That way, if it needs to speak to Machine 2 again soon, it doesn't have to
repeat the ARP request.
ARP Advice
Sometimes, ARP is the easiest way to find
a machine's ethernet address. For example, the Solaris ifconfig command will not tell you a device's ethernet address, but viewing the
ARP cache on the machine will show it to you.
The arp command
The arp command enables you to view the ARP cache, as well as to forcibly add and delete entries. We will experiment with arp
in the next exercise, after a brief discussion of the Reverse Address Resolution Protocol (RARP).
RARP
RARP is used for the opposite purpose; namely, to convert an ethernet address to an IP address. Its linux-network-administration purpose is to
enable diskless machines, such as X workstations, to find out their IP address at boot time. Such a machine broadcasts a reverse-ARP request,
such as: RARP request—My ethernet address is such and such, what should my IP address be? RARP reply—Your IP address is so and
so.
The RARP reply must come from an RARP server, which linux-network-administrationtains a file (/etc/ethers) listing the mapping from ethernet
addresses to IP addresses.
Address Resolution Protocol - Quiz
Click the Quiz link below to take a short multiple-choice quiz on ARP and RARP.
Address Resolution Protocol - Quiz
Address Resolution Protocol - Quiz
