Address Resolution Protocol (ARP): What it is and why it matters

Behind every quick file transfer or screen-mirroring connection is a system that’s quietly mapping who’s who on your network. Address Resolution Protocol (ARP) helps devices on local networks match digital names to physical locations so information gets to exactly where it’s meant to go.

In this article, we’ll help you understand how ARP keeps your network running efficiently, the different categories of this protocol, and why it’s essential to modern networking.

What is the Address Resolution Protocol (ARP)?

ARP is a network protocol used in IPv4 networks that discovers which Media Access Control (MAC) address is associated with a given IP address on the local network.

When one device wants to send information to another, it uses ARP to discover the correct hardware address for the destination IP. It’s what helps your laptop find your wireless printer, or lets your phone stream music to another device on your home Wi-Fi network.

Why is ARP important in networking?

ARP is essential because it keeps local network communication possible in IPv4 networks. Every device on a local area network (LAN) relies on ARP to send data to the right place. Without ARP, the sender (like your computer) might know who it wants to reach (your printer), but not how to get there.

How does ARP work?

ARP operates within a single LAN, like your home Wi-Fi or an office network. It works at the data-link layer (Layer 2) of the Open Systems Interconnection (OSI) model, where devices communicate directly using their MAC addresses.

When one device needs to contact another on the same LAN, ARP quietly handles the background task of finding which physical machine corresponds to a given IP address.

Step-by-step process

The ARP process follows a simple sequence of steps that help devices find and remember each other on a local network.

Sending ARP requests and receiving responses

When a device wants to send data but doesn’t yet know the MAC address of the recipient, it must follow a few steps to find it.

1. Building an ARP request: The sending device constructs an ARP request packet. The packet includes:

• The sender’s own MAC and IP addresses (so others know who’s asking)
• The target IP address of the device it’s trying to reach
• An empty target MAC address field: that’s the missing information ARP is trying to find

2. Broadcasting the ARP request: Once the request is built, the device broadcasts it to all devices on the local network. This means every device on that network segment receives the same message, which essentially asks: “Who has this IP address? Tell me your MAC address.”
3. Receiving the ARP request: Each device that receives the broadcast checks the target IP field to see if it matches its own IP address. If it doesn’t match, the device ignores the request. If it does match, that device recognizes that it’s being asked for its MAC address.
4. Sending the ARP reply: The device whose IP matches the request sends back an ARP reply directly (not broadcast) to the original sender. This reply includes the responder’s MAC address, completing the missing link between IP and hardware address.

Building and managing ARP tables

Once a device has successfully matched an IP address to a MAC address, it stores that information in an ARP table, also known as an ARP cache. It's a small database of IP and MAC address pairs it has recently learned, and every device on an IPv4 network has one.

This saves the device from having to repeat the ARP process every time it needs to send data to the same destination, which keeps communication fast and efficient. You can access your device’s ARP cache by inputting the “arp -a” command into the terminal. You’ll see something like this on the other end:

ARP entries are temporary by default. Each mapping has a timeout period, usually a few minutes, after which it’s automatically removed from the table if not refreshed. This ensures the table reflects the current state of the network and doesn’t contain outdated information.

However, network administrators can manually add static ARP entries to the cache. These don’t expire, which makes them useful for securing critical devices whose IP or MAC addresses never change (e.g. printers or routers) and isolating network problems.

There are two ways that ARP tables can be updated. The first is through a gratuitous ARP message, where a device broadcasts its own IP-to-MAC pairing to the rest of the network, typically when it joins the network for the first time. The second is through normal network updates, like when devices learn new information: for example, if a router’s interface changes or a device reports a different MAC address for a known IP.

Real-world ARP example

Imagine you’re sitting at your laptop and you hit print on a document. When you do this, your laptop needs to send the document across your home Wi-Fi network to your wireless printer.

As soon as you click print, your laptop will scan its ARP cache to find the printer’s MAC. If it can’t find the address, it will send out an ARP request using the printer’s IP address to every device on your network.

When your printer recognises its own IP in the request, it will reply with its MAC address. Your laptop receives that information and adds it to its ARP table before sending the document to the correct destination. The next time you print, that ARP lookup happens instantly and the process is much more efficient.

Advantages and limitations of ARP

ARP keeps communication on local networks running efficiently, but it isn’t without its flaws. In the sections below, we’ll explore what makes ARP so effective, what can go wrong when it fails, and how newer technologies are starting to take its place.

Benefits of ARP

It might work quietly in the background, but ARP plays a crucial role in keeping networks fast, reliable, and easy to use. Some of its biggest benefits include:

• Seamless device communication: Automatic discovery on the local network enables wireless printing, media streaming, and file sharing without manual setup.
• Ease of use: Users don’t have to configure or remember hardware addresses, making everyday connectivity simple across different devices.
• Efficient data delivery: Accurate IP to MAC mapping ensures data packets reach the correct device, reducing errors and network slowdowns.
• Minimal manual setup: Once devices are connected, ARP handles address matching automatically, so there’s no need for users to maintain or refresh settings.
• Scalability: Adding new devices is effortless, since ARP immediately recognises and integrates them into the existing network.
• Support for secure connections: Reliable address matching helps encryption tools and authentication systems confirm they’re communicating with the right device.

Without this protocol, even simple activities like browsing or streaming would demand manual address management and constant upkeep.

ARP limitations

Although ARP is simple and effective, it comes with several limitations that make it less suited for large networks with complex security needs. These weaknesses are some of the reasons why administrators often layer additional protections over ARPs:

• No built-in security: When devices exchange ARP request and reply messages to share their IP–MAC address information, ARP automatically trusts whatever it receives. It has no way to verify whether those messages are genuine, which means false or malicious data can be accepted.
• Unencrypted traffic: All ARP exchanges are sent in plain text, making them easy to intercept or alter for anyone that manages to get into the network.
• Broadcast overhead: Every ARP request is broadcast to all devices on the local network. On larger networks, this constant chatter can create unnecessary traffic and slow things down.

The protocol is far from obsolete, but these limitations do highlight why you need to implement additional security measures to keep device communication safe.

What happens when ARP fails?

When ARP fails, communication on a local network breaks down at the most fundamental level. Devices may still have valid IP addresses, but without the correct MAC address mappings, they can’t actually deliver packets to one another.

You might notice pages taking forever to load, devices dropping off the network, or your printer refusing to connect no matter how many times you try. In some cases, data simply ends up at the wrong destination or disappears altogether.

For larger networks, the fallout can be much worse. Misrouted traffic can slow entire systems, interrupt services, or open doors to attackers looking to exploit confusion in the network. When ARP fails, it doesn’t just affect one connection, it can ripple through the entire system.

Types of ARP

While the standard ARP handles most everyday communication, there are several variations designed for specific situations. Each type serves a slightly different purpose, from helping devices communicate across networks to updating information or providing identification without being prompted.

Proxy ARP

A proxy ARP helps devices on different network segments communicate as if they were on the same local network. It essentially answers ARP requests on behalf of another device, and it’s almost always a router configured to act as the proxy.

When a device sends an ARP request for an IP address outside its local subnet, the router configured for proxy ARP answers the request with its own MAC address. From the sender’s perspective, it looks like the destination is directly reachable. The device then sends its packets to the router, which forwards them to the real device on the other network.

This setup is especially useful in networks where devices are configured with overlapping subnets or when there’s no proper routing in place. That said, it can increase broadcast traffic dramatically if used too broadly.

Gratuitous ARP

Gratuitous ARP is a type of ARP broadcast that a device sends to announce or confirm its own IP-to-MAC address mapping to other devices on the network, rather than requesting that information about another device.

Devices typically send gratuitous ARPs when they connect to the network or when their IP or MAC address changes. This helps other devices update their ARP tables automatically and avoid sending packets to outdated addresses.

It’s also a simple way for a device to check if another device is using the same IP. If it gets a reply when it sends out the gratuitous ARP, there’s a conflict.

Reverse ARP

Reverse ARP, or RARP, performs the opposite function of an ARP: it maps a MAC address to an IP address. It allows a device to discover its own IP address when it only knows its MAC address. This type of ARP was common in older networks where diskless workstations (devices that didn’t have their own storage) couldn’t save configuration data.

When powered on, the device would broadcast a RARP request containing its MAC address and asking for its own IP. A RARP server on the network, preconfigured with a list of MAC-to-IP mappings, would reply with the correct IP address so the device could start communicating.

RARP has largely been replaced by the Dynamic Host Configuration Protocol (DHCP), which automates the entire IP assignment process and works more efficiently across modern networks.

Inverse ARP

Inverse ARP, or InARP, is a variation of ARP used on non-Ethernet networks such as Frame Relay or Asynchronous Transfer Mode (ATM), where devices already have a connection established at the data-link layer but don’t yet know each other’s IP addresses.

Instead of starting with an IP address like standard ARP, InARP begins with a known data link connection identifier and essentially asks, “What IP address is associated with this link?”

When the connection comes up, each end sends an InARP Request across it. The remote device replies with its IP address in an InARP Reply, which allows both sides to automatically learn each other’s IPs without needing static configuration.

InARP is especially useful in non-broadcast networks, where devices can’t send requests to everyone on the network. Today, it’s mostly obsolete since Ethernet and IP-based routing have replaced those older technologies.

ARP in the context of other protocols

Every network runs on a collection of communication protocols that interact across different layers of the OSI model. The model divides the data journey into seven layers, each with a specific role. These layers don’t work in isolation but depend on one another, so information has to be passed up and down the stack to make sure the right data reaches the right place in the right form.

ARP vs. DHCP vs. DNS

ARP, DHCP, and Domain Name System (DNS) all deal with network addressing, but they operate at different layers and serve distinct purposes. Together, they form part of the invisible foundation that allows devices and websites to talk to one another.

• ARP works at the data-link layer (Layer 2): It maps an IP address to a MAC address so data can be delivered on the local network.
• The DHCP works at the application layer (Layer 7): It assigns IP addresses to devices when they join a network. This saves users from having to set up addresses manually and ensures that no two devices accidentally get the same one. It replaced older systems like RARP, which once handled this task in much simpler, less flexible ways.
• DNS also works at the application layer: It acts like the internet’s phone book, taking human-readable web addresses and aligning them with corresponding IP addresses that computers can understand.

When and where ARP is used in networks

ARP is active whenever devices on a local network need to find each other and plays an important role in a variety of everyday and technical situations, including:

• LANs: Helps devices on the same Ethernet or Wi-Fi LAN in homes, offices, and schools to communicate directly.
• Device discovery: Enables computers, printers, and routers to identify each other by linking IP and MAC addresses.
• Data delivery: Ensures that data packets reach the correct physical device within the network instead of getting lost or misrouted.
• File and resource sharing: Supports smooth communication when transferring files, streaming media, or using shared drives and servers.
• Network maintenance: Keeps routing tables accurate and ensures ongoing communication efficiency between connected devices.

ARP spoofing and network threats

ARP was built for simplicity, not security. It assumes that every ARP message it receives is genuine, without checking whether the sender is who it claims to be. This design makes communication fast and seamless but also leaves the protocol vulnerable.

What is ARP spoofing?

ARP spoofing, also called ARP poisoning, ARP cache poisoning, and ARP poison routing, is a type of cyberattack is a type of attack where a malicious actor sends fake ARP messages on a local network.

The goal is to associate the attacker’s MAC address to the IP address of another legitimate device, such as a router or server. Once this false mapping is accepted, traffic meant for the real device is instead sent to the attacker.

This is possible because ARP’s biggest weakness is trust: it automatically accepts and caches any believable IP-to-MAC mapping it sees, without authenticating the source. As a result, forged or unsolicited ARP messages, including fake gratuitous ARPs, can overwrite correct entries and redirect traffic to an attacker.

Once an attacker succeeds, several types of attacks can follow, and these are the most common ones:

Man-in-the-middle attacks

A man-in-the-middle (MITM) attack happens when an attacker secretly positions themselves between two communicating devices and intercepts the data being exchanged.

This is easy to do with ARP spoofing. The attacker simply alters the ARP cache to convince both devices that they’re sending data directly to each other, when in fact every message passes through the attacker first. They can then monitor, change, or redirect traffic without either device noticing.

A well-known MITM variation is a secure Sockets Layer (SSL) stripping attack. where the attacker downgrades a secure HTTPS connection to unencrypted HTTP. This allows sensitive data like passwords or payment information to be viewed in plain text. Modern defenses like HTTP Strict Transport Security (HSTS), proper Transport Layer Security (TLS) configuration, and browser protections, make SSL-stripping far more difficult to execute against correctly configured websites.

Denial-of-service (DoS) attacks

A DoS attack aims to overwhelm or disrupt a device or network so that legitimate users can’t access it.

In the context of ARP spoofing, attackers flood the network with false ARP replies, forcing devices to constantly update their ARP tables or send data to the wrong destination. This overload can slow network performance to a crawl or make it impossible to reach local devices.

Even short-lived DoS attacks can expose deeper vulnerabilities in a network. When devices are overwhelmed or misdirected, attackers can sometimes use the chaos to slip in unnoticed, install malware, or launch follow-up attacks.

Session hijacking

A session hijacking attack happens when an attacker takes over an active connection between two devices, often by stealing or guessing a valid session token.

With ARP poisoning, attackers can intercept traffic in a local network and insert themselves into the communication stream to impersonate one of the devices involved. Once they’ve gained control, they can access private accounts, send unauthorised commands, or harvest sensitive information without being detected.

A popular variation of session hijacking is DNS address hijacking. This is where users are silently redirected to a fake version of a trusted website. Once there, attackers can capture login details or other sensitive information before they’re forwarded to the legitimate site.

Both of these hijacking attacks can potentially expose users to data theft, identity fraud, and financial loss.

How to detect ARP spoofing

ARP spoofing often happens silently in the background, so you’ll need to be able to spot unusual network behaviour and verify address information to detect it. The steps below can help you identify potential problems before they escalate:

1. Check for early tell-tale symptoms first: Sudden latency, HTTPS warnings, or frequent disconnects on a LAN can hint at poisoned ARP caches.
2. Inspect your ARP table: Run the “arp -a” command on your device’s terminal and look for one MAC mapped to many IPs, or rapid changes in entries that should stay stable.
3. Capture traffic and filter for anomalies: Use a network protocol analyzer to filter for anomalies such as duplicate address frames or conflicting IP claims.
4. Watch for unsolicited ARP replies or floods: A high volume of ARP responses that weren’t requested is a classic red flag.
5. Set up ARP change alerts: Use a network monitoring tool that can log IP-to-MAC pairings and alert you when new or unexpected associations appear.

ARP spoofing prevention techniques

To prevent ARP spoofing, you’ll need to tighten your network controls and use tools that verify who’s really sending data. There are two main prevention techniques for large or mid-size networks: Dynamic ARP Inspection (DAI) and Port Security.

DAI is a security feature built into many managed switches that checks each ARP message against a trusted database of valid IP-to-MAC pairings. It’s often used alongside Port Security, which limits which MAC addresses can appear at each switch port. Together, these features validate ARP traffic against trusted DHCP bindings and block any suspicious or unauthorized devices before they can cause a disruption in the network.

For home users, keeping systems updated and using static ARP entries for critical devices (e.g. routers and printers) can help to minimize the risk. Limiting who can access your local network also makes it harder for attackers to inject false ARP messages in the first place.

Tools to monitor and analyze ARP traffic

Keeping an eye on ARP traffic can help you spot unusual activity before it turns into a problem. The tools below range from lightweight utilities to full-scale network monitoring platforms.

ARP troubleshooting

If you find that your network is slow or devices have stopped communicating with one another, the problem may lie with your ARP.

Common ARP issues often stem from cache errors, duplicate IP addresses, or outdated information in ARP tables. All of which can cause traffic to go to the wrong destination. Here are a few steps you can take to remedy the issue:

1. Clear the ARP table: This forces a device to rebuild its cache with fresh data and often resolves temporary conflicts. On most systems, you can use the command line using “arp -d” followed by the IP address.
2. Check for duplicate IPs: Run “arp -a” to compare IP to MAC pairings across devices. If multiple devices show the same IP, disconnect one and test again to confirm the conflict. You can also temporarily assign static IPs to critical devices to avoid DHCP overlaps.
3. Verify network device settings: Misconfigured routers or switches can serve incorrect ARP data. Restart your router, check that DHCP settings are correct, and ensure firmware is up to date.
4. Rebuild connections: Renew your device’s IP address to trigger a new ARP resolution. Use “ipconfig /renew” on Windows devices, “dhclient” on Linux, and “sudo ipconfig set en0 DHCP” on macOS.

Alternatives to ARP in modern networks

Although ARP is still the standard for linking IP and MAC addresses on IPv4 networks, there are newer systems that have been designed to handle the same job in more modern setups.

For IPv6 networks, that’s the Neighbor Discovery Protocol (NDP). It does pretty much the same for IPv6 as ARP does for IPv4, with just a few upgrades: instead of simple broadcasts, NDP It uses ICMPv6 messages, which are more efficient than broadcasts because they target specific devices instead of flooding the network, and they support built-in authentication and duplicate-address detection, making communication faster and more secure.

In cloud environments and virtual networks, things work a bit differently. Instead of physical devices broadcasting ARP messages across a local network, software-defined networking (SDN) systems manage the entire process.

These systems maintain virtual versions of ARP tables and automatically map virtual machine addresses to their underlying hardware. Virtual switches, which connect these machines within a shared server, ensure that data gets where it needs to go without flooding the network with unnecessary ARP requests.