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ARP Message Format

ARP Message Format
Address resolution using ARP is accomplished through the exchange of messages between the source device seeking to perform the resolution, and the destination device that responds to it. As with other protocols, a special message format is used containing the information required for each step of the resolution process.
ARP messages use a relatively simple format. It includes a field describing the type of message (its operational code or opcode) and information on both layer two and layer three addresses. In order to support addresses that may be of varying length, the format specifies the type of protocol used at both layer two and layer three and the length of addresses used at each of these layers. It then includes space for all four of the address combinations we saw in the previous topic.
The format used for ARP messages is described fully in Table 42, and illustrated in Figure 49.

Table 42: Address Resolution Protocol (ARP) Message Format
Field Name
Size (bytes)
Description
HRD
2

PRO
2
Protocol Type: This field is the complement of the Hardware Type field, specifying the type of layer three addresses used in the message. For IPv4 addresses, this value is 2048 (0800 hex), which corresponds to the EtherType code for the Internet Protocol.
HLN
1
Hardware Address Length: Specifies how long hardware addresses are in this message. For Ethernet or other networks using IEEE 802 MAC addresses, the value is 6.
PLN
1
Protocol Address Length: Again, the complement of the preceding field; specifies how long protocol (layer three) addresses are in this message. For IP(v4) addresses this value is of course 4.
OP
2

SHA
(Variable, equals value in HLNfield)
Sender Hardware Address: The hardware (layer two) address of the device sending this message (which is the IP datagram source device on a request, and the IP datagram destination on a reply, as discussed in the topic on ARP operation).
SPA
(Variable, equals value in PLNfield)
Sender Protocol Address: The IP address of the device sending this message.
THA
(Variable, equals value in HLNfield)
Target Hardware Address: The hardware (layer two) address of the device this message is being sent to. This is the IP datagram destination device on a request, and the IP datagram source on a reply)
TPA
(Variable, equals value in PLNfield)
Target Protocol Address: The IP address of the device this message is being sent to.

Comments

Unknown said…
Hi Fazil,

Jeez oh man, while I applaud for your writing, it’s just so damn straight to the point ARP Message Format.

I have a net book Samsung N150plus, and sometimes I’m having problems with google that crashes and close, even is a bit slow as I’m going to a 2gb ram also, I had re installed it several times but someone told me to install a Linux software on my net book,
can I? Are there any disadvantages?
In that case your server will be in unbootable state. Your Server can’t boot without /boot directory because this directory contains all bootable files.
Please keep providing such valuable information.

Obrigado,
Kevin
May 19, 2018 at 2:42 AM
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