Address Resolution Protocol Tutorial, How ARP work, ARP Message Format

Address Resolution Protocol (ARP) is one of the major protocol in the TCP/IP suit and the purpose of Address Resolution Protocol (ARP) is to resolve an IP address (32 bit Logical Address) to the physical address (48 bit MAC Address). Network Applications at the Application Layer use IP address to communicate with another device. But at the Datalink layer, the addressing is MAC address (48 bit Physical Address), and this address is burned into the network card permanently. You can view your network card’s hardware address by typing the command "ipconfig /all" at the command prompt (Without double quotes using Windows Operating Systems).

The purpose of Address Resolution Protocol (ARP) is to find out the MAC address of a device in your Local Area Network (LAN), for the corresponding IP address, which network application is trying to communicate.

Address Resolution Protocol (ARP) Message Format

Address Resolution Protocol ARP Message Format

Address Resolution Protocol (ARP) Message Format

Following are the fields in the Address Resolution Protocol (ARP) Message Format.

Hardware Type: Hardware Type field in the Address Resolution Protocol (ARP) Message specifies the type of hardware used for the local network transmitting the Address Resolution Protocol (ARP) message. The value for Ethernet is 1. The size of this field is 2 bytes.

Protocol Type: Each protocol is assigned a number used in this field. IPv4 is 2048 (0x0800 in Hexa).

Hardware Address Length: Hardware Address Length in the Address Resolution Protocol (ARP) Message is length in bytes of a hardware (MAC) address. Ethernet addresses are 6 bytes long.

Protocol Address Length: Length in bytes of a logical address. IPv4 addresses are 4 bytes long.

Opcode: Opcode field in the Address Resolution Protocol (ARP) Message specifies the nature of the ARP message. 1 for ARP request and 2 for ARP reply.

Sender Hardware Address: Layer 2 (MAC Address) address of the device sending the message.

Sender Protocol Address: The protocol address (IP address) of the device sending the message

Target Hardware Address: Layer 2 (MAC Address) of the intended receiver. This field is ignored in requests.

Target Protocol Address: The protocol address (IP Address) of the intended receiver.

Working of Address Resolution Protocol (ARP)

Step 1: When a source device want to communicate with another device, source device checks its Address Resolution Protocol (ARP) cache to find it already has a resolved MAC address of the destination device. If it is there, it will use that address for communication. To view your Local Address Resolution Protocol (ARP) cache, Open Command Prompt and type command "arp -a" (Without double quotes using Windows Operating Systems).

Step 2: If ARP resolution is not there in local cache, the source machine will generate an Address Resolution Protocol (ARP) request message, it puts its own data link layer address as the Sender Hardware Address and its own IP address as the Sender Protocol Address. It fills the destination IP address as the Target Protocol Address. The Target Hardware Address will be left blank, since the machine is trying to find that.

Step 3: The source broadcast the Address Resolution Protocol (ARP) request message to the local network.

Step 4: The message is received by each device on the LAN since it is a broadcast. Each device compare the Target Protocol Address (IP Address of the machine to which the source is trying to communicate) with its own Protocol Address (IP Address). Those who do not match will drop the packet without any action.

Step 5: When the targeted device checks the Target Protocol Address, it will find a match and will generate an Address Resolution Protocol (ARP) reply message. It takes the Sender Hardware Address and the Sender Protocol Address fields from the Address Resolution Protocol (ARP) request message and uses these values for the Targeted Hardware Address and Targeted Protocol Address of the reply message.

Step 6: The destination device will update its Address Resolution Protocol (ARP) cache, since it need to contact the sender machine soon.

Step 7: Destination device send the Address Resolution Protocol (ARP) reply message and it will not be a broadcast, but a unicast.

Step 8: The source machine will process the Address Resolution Protocol (ARP) reply from destination, it store the Sender Hardware Address as the layer 2 address of the destination.

Step 9: The source machine will update its Address Resolution Protocol (ARP) cache with the Sender Hardware Address and Sender Protocol Address it received from the Address Resolution Protocol (ARP) reply message.

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You have learned Address Resolution Protocol (ARP), Address Resolution Protocol (ARP) Message Format and how Address Resolution Protocol (ARP) operate in a LAN. Click "Next" to continue.