ARP Request Packet: A Beginner's Guide 🚀
Published: 2023-05-30 21:42:36



ARP stands for Address Resolution Protocol, which is a protocol that maps an IP address to a physical address, such as an Ethernet address. ARP is used when a device wants to communicate with another device on the same network, but does not know its physical address. 🙌

In this article, we will learn about the ARP request packet, which is a packet that is sent by a device to ask for the physical address of another device with a given IP address. We will also see how to encapsulate the ARP request packet in a data link frame, which is a unit of data that is transmitted over a physical medium, such as an Ethernet cable. 🧐

## What is an ARP Request Packet? 🤔

An ARP request packet is a packet that contains the following information:

- The sender's IP address and physical address.
- The target's IP address and an unknown physical address.
- The type of protocol used, such as IPv4 or IPv6.
- The type of hardware used, such as Ethernet or Wi-Fi.
- The operation code, which indicates whether the packet is a request or a reply.

An ARP request packet is broadcasted to all devices on the same network, and only the device with the matching IP address responds with an ARP reply packet that contains its physical address. The sender then updates its ARP cache, which is a table that stores the mappings between IP addresses and physical addresses. 📧

## How to Encapsulate an ARP Request Packet in a Data Link Frame? 📝

A data link frame is a unit of data that is transmitted over a physical medium, such as an Ethernet cable. A data link frame has a header and a trailer that contain information such as the source and destination physical addresses, the type of data, and the error detection code.

To encapsulate an ARP request packet in a data link frame, we need to follow these steps:

- Add the source and destination physical addresses to the header of the frame. The source physical address is the sender's Ethernet address, and the destination physical address is the broadcast address (FF:FF:FF:FF:FF:FF), which means that the frame will be sent to all devices on the network.
- Add the type of data to the header of the frame. The type of data indicates what kind of protocol is used in the payload of the frame. For ARP packets, the type of data is 0x0806.
- Add the payload of the frame, which is the ARP request packet itself. The payload contains the sender's IP address and physical address, the target's IP address and unknown physical address, the type of protocol used (0x0800 for IPv4), the type of hardware used (0x0001 for Ethernet), and the operation code (0x0001 for request).
- Add the trailer of the frame, which contains an error detection code called cyclic redundancy check (CRC). The CRC is a mathematical function that calculates a value based on the bits in the frame. The CRC helps to detect any errors or changes in the frame during transmission.



## Example 🤷‍♂️

Let's see an example of how to encapsulate an ARP request packet in a data link frame.

Suppose we have a router with IP address 125.45.23.12 and Ethernet physical address 23:45:AB:4F:67:CD that has received a packet for a destination with IP address 125.11.78.10 and Ethernet physical address AA:BB:A2:4F:67:CD.

The router does not know the Ethernet physical address of the destination, so it needs to send an ARP request packet to ask for it.

Here are the entries in the ARP request packet sent by the router:

| Field | Value |
| ----- | ----- |
| Sender IP Address | 125.45.23.12 |
| Sender Physical Address | 23:45:AB:4F:67:CD |
| Target IP Address | 125.11.78.10 |
| Target Physical Address | 00:00:00:00:00:00 |
| Protocol Type | 0x0800 |
| Hardware Type | 0x0001 |
| Operation Code | 0x0001 |

Here are the entries in the data link frame that encapsulates the ARP request packet:

| Field | Value |
| ----- | ----- |
| Source Physical Address | 23:45:AB:4F:67:CD |
| Destination Physical Address | FF:FF:FF:FF:FF:FF |
| Type of Data | 0x0806 |
| Payload | ARP Request Packet |
| CRC | Calculated Value |



## Conclusion 🎉

In this article, we learned about:

- What is an ARP request packet and why it is important.
- How to encapsulate an ARP request packet in a data link frame and what are the fields involved.
- An example of how to encapsulate an ARP request packet in a data link frame.

We hope you enjoyed this article and learned something new about ARP! 😊

If you have any questions or feedback, please leave them in comments below! 👇