18 January 2022 | 5 min read | Ignas Anfalovas
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What Is Address Resolution Protocol? A Beginner’s Guide to ARP

There are different types of Address Resolution Protocol. This article defines them, explains why ARP is important and how it works, as well as unravels ARP spoofing.

Two computers exchanging ARP queries and responses.

The Address Resolution Protocol (ARP) is a communication protocol that maps the Internet Protocol (IP) address to the Media Access Control (MAC) address. This protocol facilitates the communication of the devices connected to the network. 

Applications and software connected to the internet use IP addresses to send information. Meanwhile, the communication between systems happens through hardware addresses, also known as MAC or physical addresses. Without ARP, software and devices would not be able to send data to each other. 

Continue reading the article to understand how ARP enables communication between devices on the network and why it is important.  

Why is ARP important?

ARP translates the software address (IP address) to the physical address (MAC address) of the host connected to the network. ARP exists as the link layer protocol in the Open System Interconnection (OSI) model. 

The conceptual OSI model defines the standard of communication between different computer systems. This model uses layers to visualize how various systems employ standard protocols in their communication with each other. 

In the OSI model, MAC addresses are a sublayer within the data link layer (therefore, known as data link addresses). The data link layer is responsible for maintaining the connection between two physically connected devices and allows them to transfer data. Meanwhile, IP addresses belong to the network layer. The network layer forwards packets through different routers. 

However, destination decisions can’t be based on IP addresses. On the same network, an IP address maps the data link layer address of another computer.  

Here’s where ARP comes into play. Since IP version 4 (IPv4) addresses have different lengths (32-bit) than MAC addresses (48-bit), ARP translates these addresses to facilitate the information exchange.

Bit size and character length comparison of MAC and IPv4 addresses.
MAC address vs. IPv4 address

Essentially, without ARP, a host would not be able to figure out the hardware address of another host.

How does ARP work? 

When a source device wants to send an IPv4 packet to another device, ARP performs two important tasks. First, the ARP program checks the ARP cache table, which consists of IPv4 address to MAC address mappings. 

The second task starts if the ARP cache lookup doesn’t provide a matching MAC address. In this case, the source server forms an ARP message, which is broadcast on the local area network (LAN). 

Let’s discuss how ARP requests work in greater detail. 

ARP request

An ARP request establishes communication between devices on the network. It is enabled after a source device fails to retrieve necessary data from an ARP cache table

The ARP table holds records of the IP address and MAC address of the devices connected to the same network. IT administrators do not maintain this table. Instead, the ARP protocol creates additions when it receives an ARP response. All operating systems in a network keep ARP caches.

One computer in a network sends a request and receives a response.
How ARP works using a request-response system

It’s worth noting that the ARP cache size is small. That’s why the cache periodically cleanses itself to free space for new entries. 

Here’s a detailed example of the ARP process. 

Let’s say there are two computers in a local area network psychically connected by Ethernet cables and network switches, without gateways or routers that intervene. One of the computers, let’s call it Host 1, receives a command to send a packet to the other computer – Host 2. 

To reach the destination, Host 1 uses DNS to determine the unique IP address of Host 2. However, it also needs a hardware address to send the packet. 

First of all, Host 1 has to check a cached ARP table to look up the IP address for any existing records of the target’s MAC address. If the cached records exist, then Host 1 sends the IP packet through an Ethernet frame (a destination MAC address delivers this frame).

If the hardware address was not available in the cache, Host 1 then sends the ARP protocol that contains an ARP broadcast packet. Host 2 reacts by sending an ARP response message with IP and MAC addresses.

After receiving the ARP response, Host 1 updates its ARP table with the new information and delivers the IP packets.

Types of ARP

There are four types of Address Resolution Protocol:

  • Proxy ARP
  • Gratuitous ARP
  • Reverse ARP
  • Inverse ARP
Proxy ARP, Gratuitous ARP, Reverse ARP and Inverse ARP listed as types of ARP.
Types of ARP

Let’s take a closer look at each of them.

Proxy ARP

A Proxy ARP protocol handles requests coming from IP addresses that aren’t on the local area network. When the request packet comes from a system outside the host’s network, the protocol configured router responds to the ARP request packet. 

However, instead of providing a MAC address of the target host, the router responds by pretending to be the destination and delivers its own MAC address instead. 

Gratuitous ARP

A Gratuitous ARP works as an ARP response that is not evoked by an ARP request. It assists when a host needs to announce or update its IP address to MAC address mapping to the entire network.

Reverse ARP

A Reverse Address Resolution Protocol (RARP) requests IPv4 addresses from the ARP gateway router table. New systems or machines that don’t have the memory to store their own IP addresses tend to use this type of ARP. 

The requesting device broadcasts a Reverse ARP packet with its own MAC address in the address field of both the sender and the receiver hardware. The RARP server installed in the local network responds to the request. The server then locates a mapping table entry in the IP to MAC address. If the matching is successful, the server sends the response packet with the IP address of the requesting computer. 

Inverse ARP

An Inverse ARP (InARP) does the opposite of what a traditional ARP does – it maps IP addresses using the associated hardware addresses. InARP assists when a device knows the Data Link Connection Identifier (DLCI) of a remote router but does not know its own IP address.

What is ARP spoofing? 

Also known as ARP cache poisoning or ARP poison routing, ARP spoofing is a malicious cyberattack that cybercriminals are responsible for. During ARP spoofing attacks, fake ARP messages go out to a target LAN. 

This enables an attacker to link their MAC address with the IP address of a device on the network. If the ARP poisoning attack is successful, the criminal’s computer receives data from the victim’s computer.

A diagram of a hacker spoofing ARP to cause harm to a targeted victim.
How ARP spoofing works

Needless to say, these data breaches can cause serious harm and are acted out without the computer’s owner even noticing. 

ARP spoofing can also make it easier to carry out other attacks, including:

  • Denial-of-Service (DoS) attacks
  • Man-in-the-Middle (MTM) attacks
  • Session hijacking

Attackers often use a denial-of-service attack to overwhelm servers, computer networks and systems by flooding them with traffic. A large-scale DoS attack is known as a distributed denial-of-service (DDoS) attack. DDoS uses a much greater number of resources to flood the system and prevent users from accessing it. 

When a large number of ARP packets are transmitted through a vulnerable network, the service can become overwhelmed. 

Another common attack is called man-in-the-middle. During this attack, a third party – a cyberattacker – finds a way to intercept between two other parties and steal information.

ARP poisoning can also lead to session hijacking attacks. They occur when a cybercriminal takes over the user’s web session after stealing their session ID

Conclusion

The Address Resolution Protocol (ARP) is an important protocol that can map IP addresses to MAC addresses. It facilitates communication between devices on the same network. Without ARP, software and devices would not be able to send data to each other. 

ARP has several important types: Proxy ARP, Gratuitous ARP, Reverse ARP and Inverse ARP. All of these types serve as communication protocols on the local area network. 

Unfortunately, the ARP protocol is vulnerable to spoofing, a type of malicious cyberattack performed by cybercriminals. Security measures should be taken to ensure ARP security.


About the author

Ignas Anfalovas

Platform Engineering Manager

Ignas is a Platform Engineering Manager at IPXO with more than 7 years of experience in the IT sector. His expertise includes network design solutions and infrastructure maintenance. After working hours, you will find Ignas in Lithuanian folk-dance classes.