What Is the Address Resolution Protocol (ARP)?

Address Resolution Protocol (ARP) is a communication protocol used in Internet Protocol (IP) networks to discover the Media Access Control (MAC) address of a device associated with a specific IP address. ARP operates at the link layer (Layer 2) of the OSI (Open Systems Interconnection) model, facilitating communication between devices on the same network segment.

How Does ARP Work?

When a device on a LAN needs to send a packet to another device with a known IP address but an unknown MAC address, it initiates an ARP request. This request is a broadcast message sent to all devices on the LAN, containing the target device’s IP address. Devices receiving the request will compare the target IP address with their own. If a device finds a match, it will send an ARP response containing its MAC address to the requesting device.

The requesting device stores the received MAC address in its ARP cache, a temporary storage space for IP-to-MAC address mappings. The device can then use the MAC address to send packets directly to the target device over Ethernet. If the mapping is not found in the ARP cache, the device must initiate a new ARP request.

What Is the Purpose of ARP in Networking?

The primary purpose of ARP is to map IP addresses to their corresponding MAC addresses, enabling devices on the same network segment to communicate with each other. IP addresses are used at the network layer (Layer 3) to route packets between networks, while MAC addresses are used at the link layer (Layer 2) to deliver packets within the same network segment.

What Are the Types of ARP?

There are several types of ARP, including:

  • Gratuitous ARP: Gratuitous ARP is an unsolicited ARP response sent by a device to announce its IP and MAC addresses to the entire network. This helps in detecting IP address conflicts, updating ARP tables, and informing network devices about changes in hardware addresses.
  • Reverse ARP: Reverse ARP (RARP) allows a device to discover its own IP address when it only knows its MAC address. This protocol is now considered obsolete, as it has been replaced by the Dynamic Host Configuration Protocol (DHCP).
  • Inverse ARP: Inverse ARP is used in Frame Relay and Asynchronous Transfer Mode (ATM) networks to discover the IP address associated with a specific virtual circuit.
  • Proxy ARP: Proxy ARP occurs when a router or another network device responds to ARP requests on behalf of another device, usually on a different subnet. This enables devices on separate subnets to communicate as if they were on the same network segment.

What Is the Structure of the ARP Header?

The ARP header contains the following fields:

  • Hardware type: Specifies the type of hardware used for the MAC address.
  • Protocol type: Specifies the type of protocol used for the IP address.
  • Hardware address length: Indicates the length of the MAC address.
  • Protocol address length: Indicates the length of the IP address.
  • Operation: Specifies the type of ARP message (request or response).
  • Sender hardware address: The MAC address of the device sending the ARP message.
  • Sender protocol address: The IP address of the device sending the ARP message.
  • Target hardware address: The MAC address of the target device (filled in by the target device in the ARP response).
  • Target protocol address: The IP address of the target device.

How Does ARP Maintain a Cache Table?

ARP cache is a temporary storage space in the memory of a device where it stores the recently resolved IP-to-MAC address mappings. When a device needs to communicate with another device, it first checks its ARP cache for an existing mapping. If the mapping is not found, the device initiates an ARP request. ARP cache entries have a time-to-live (TTL) value, which determines how long the mapping stays in the cache before being removed.

What Is the Process of ARP Request and ARP Reply?

The ARP request process begins when a device wants to communicate with another device on the same network but does not know its MAC address. The requesting device sends a broadcast message containing the target device’s IP address. All devices on the network receive this message.

The ARP reply process occurs when the target device with the matching IP address responds to the ARP request. It sends a unicast message back to the requesting device, containing its MAC address. The requesting device then stores this information in its ARP cache for future use.

What Is the Difference Between ARP and Reverse ARP (RARP)?

ARP is used to discover the MAC address associated with a known IP address, whereas Reverse ARP (RARP) is used to find the IP address associated with a known MAC address. RARP is now considered obsolete, as it has been replaced by more advanced protocols like DHCP.

Are There Any Limitations or Drawbacks of ARP?

There are some limitations and drawbacks associated with ARP:

  • Broadcast traffic: ARP requests are broadcast messages, which can contribute to network congestion in large networks.
  • Cache limitations: ARP cache entries have a limited lifespan, and the cache can become full, requiring the removal of older entries.
  • Security vulnerabilities: ARP is vulnerable to spoofing and poisoning attacks, which can lead to data theft or network disruption.
  • Scalability: ARP is designed for relatively small networks, and its performance can degrade in larger environments with many devices.

How Can ARP Be Used in a Malicious Way?

ARP spoofing, also known as ARP poisoning, is a type of cyberattack in which an attacker sends fake ARP messages to a network, causing devices to associate the attacker’s MAC address with a legitimate IP address. This enables the attacker to intercept or modify network traffic, acting as a man-in-the-middle. This malicious activity can lead to data theft, network disruption, or other security issues.

What Are Some Methods to Prevent ARP Related Security Issues?

There are several methods to protect against ARP spoofing and other ARP-related security issues:

  • Static ARP entries: Manually configuring devices with static IP-to-MAC address mappings can prevent attackers from injecting false ARP messages.
  • Dynamic ARP Inspection (DAI): This security feature on network switches validates ARP messages against a trusted database, filtering out any malicious ARP packets.
  • IP Source Guard: This network feature checks the source IP address of incoming packets against a trusted database, blocking traffic from untrusted sources.
  • Encryption: Using encrypted communication protocols like HTTPS and VPNs can help protect data even if an attacker successfully performs an ARP spoofing attack.

What Is the History of ARP?

ARP was first introduced in the early 1980s in the context of IPv4 networking. It was defined in RFC 826 by David C. Plummer, who proposed the protocol to enable devices on a LAN to communicate using IP addresses. ARP has since become a standard networking protocol and an essential component of IPv4 networks.

Ready to go Passwordless?

Indisputable identity-proofing, advanced biometrics-powered passwordless authentication and fraud detection in a single application.