Layer 2 – relating IP and MAC addresses using ARP

Introduction

In computer networks, layer 2 and layer 3 protocols play an important role in communication. Layer 2 protocols deal with the data transmission over a single link while layer 3 protocols are responsible for routing data between multiple networks. In this article, we will focus on layer 2 and the concept of relating IP and MAC addresses using Address Resolution Protocol (ARP).

What is ARP?

ARP stands for Address Resolution Protocol, which is a protocol used to map an IP address to a physical (MAC) address on a local network. This is essential because the data link layer (layer 2) uses MAC addresses to identify devices on a network, while the network layer (layer 3) uses IP addresses. ARP is used to translate between the two types of addresses so that data can be transmitted from one device to another.

How does ARP work?

When a device wants to send data to another device on the same network, it first needs to find out the MAC address of the destination device. The device sends an ARP broadcast request to all devices on the network asking for the MAC address of the destination device. The ARP request contains the IP address of the destination device. The device with that IP address will respond with its MAC address, which is then used by the sender to send the data.

The ARP cache

The ARP cache is a table maintained by each device on a network that stores mappings of IP addresses to MAC addresses. Each time a device sends or receives data using ARP, the information is added to the ARP cache. This helps to speed up future communication between the same devices, as the device will not need to send another ARP request for the same IP address.

ARP Spoofing

ARP spoofing is a type of attack in which an attacker sends fake ARP messages to map their MAC address to the IP address of another device on the network. This allows the attacker to intercept, modify or even block data being sent to the target device. ARP spoofing is a serious security threat and can be used to perform man-in-the-middle attacks or to steal sensitive information. To protect against ARP spoofing, it is important to use security measures such as firewalls and switch port security.

Examples

To help illustrate the concept of ARP and how it works, let’s consider the following example.

Example 1

Imagine we have two devices on a network: Device A and Device B. Device A wants to send data to Device B. Device A has the IP address 192.168.1.100 and Device B has the IP address 192.168.1.101. To send data to Device B, Device A needs to know the MAC address of Device B.

Step 1: Device A sends an ARP request to all devices on the network asking for the MAC address of the IP address 192.168.1.101.

Step 2: Device B receives the ARP request and recognizes its IP address in the request. It then responds with its MAC address of 00:11:22:33:44:55.

Step 3: Device A adds the mapping of IP address 192.168.1.101 to MAC address 00:11:22:33:44:55 to its ARP cache.

Step 4: Device A uses the MAC address 00:11:22:33:44:55 to send data to Device B.

Example 2

In this example, we have three devices on a network: Device A, Device B, and Device C. Device A wants to send data to Device C, but it doesn’t have the MAC address of Device C in its ARP cache.

Step 1: Device A sends an ARP request to all devices on the network asking for the MAC address of the IP address of Device C, which is 192.168.1.103.

Step 2: Device B and Device C both receive the ARP request, but only Device C recognizes its IP address in the request. It then responds with its MAC address of 66:77:88:99:AA:BB.

Step 3: Device A adds the mapping of IP address 192.168.1.103 to MAC address 66:77:88:99:AA:BB to its ARP cache.

Step 4: Device A uses the MAC address 66:77:88:99:AA:BB to send data to Device C.

Conclusion

In this article, we have discussed the role of ARP in relating IP and MAC addresses in computer networks. We have also discussed the working of ARP, the ARP cache, and the security threat of ARP spoofing. With the help of examples, we have seen how ARP can be used to send data from one device to another on a network. Understanding the concept of ARP is important for network administrators as it helps them to troubleshoot network issues and improve network security.

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