Purpose of Address Resolution Protocol

Lesson 10

Objective

ARP and RARP
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 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 button to take a short multiple-choice quiz on ARP and RARP.
Address Resolution Protocol - Quiz


	Lesson 10 Objective	ARP and RARP 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 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 button to take a short multiple-choice quiz on ARP and RARP. Address Resolution Protocol - Quiz