What is the Address Resolution Protocol (ARP)?

What is ARP and how does it work?

The Address Resolution Protocol (ARP) is a communication protocol used for discovering the link-layer address (MAC address) that corresponds to a given network-layer address (IP address). It’s an essential part of the IPv4 networking stack and is used to enable communication between devices on the same local area network (LAN).

When a device wants to communicate with another device on the LAN, it knows the target’s IP address (e.g., 192.168.1.5) but not the MAC address required to send data at the data link layer. To resolve this, the sender sends out an ARP request—a broadcast message that asks, “Who has IP address 192.168.1.5?” Every device on the LAN sees this message, but only the device with that IP address responds, replying with its MAC address (e.g., 00:1A:2B:3C:4D:5E). The sender then stores this IP-to-MAC mapping in its ARP cache for future use.

ARP operates only within a local network segment. If the destination IP is on another subnet, the packet is forwarded to a router, and ARP is used to find the MAC address of that router’s interface instead.

There are a few ARP variants worth noting:

• Gratuitous ARP: A device announces its IP-MAC mapping, often used to detect IP conflicts or update caches.

• Proxy ARP: A router replies to ARP requests on behalf of another device, making it seem local.

• Reverse ARP (RARP): Now obsolete, this was used to find an IP address when only the MAC address was known.

ARP is simple but critical. Without it, devices wouldn’t know how to deliver packets at the data link layer—rendering local communication impossible.

What is the difference between ARP and DNS?

ARP and DNS are both resolution protocols, but they operate in completely different layers of the networking stack and serve very different purposes.

DNS (Domain Name System) is responsible for resolving human-readable domain names (like www.example.com) into IP addresses (like 93.184.216.34). It operates at the application layer of the OSI model and is designed to make the internet user-friendly. DNS is used globally, across wide-area networks, and supports features like load balancing, caching, and failover.

ARP, on the other hand, is used to resolve IP addresses to MAC addresses. It operates at the link layer (between network and data link layers) and is confined to local area networks. Its role is to enable devices within the same subnet to communicate using Ethernet or Wi-Fi.

In short:

• DNS: Name → IP (used across the internet)

• ARP: IP → MAC (used within local networks)

While both are resolution tools, ARP is like asking, “How do I reach that specific machine on my local street?” while DNS is more like asking, “What’s the street address of Starbucks?”

What is ARP spoofing and how can it be prevented?

ARP spoofing (or ARP poisoning) is a cyberattack in which a malicious actor sends fake ARP messages over a local network. The goal is to associate the attacker’s MAC address with the IP address of a legitimate device, often a default gateway, so that traffic intended for that IP is redirected to the attacker.

Here’s how it works: Suppose a victim device wants to send data to the network gateway at IP 192.168.1.1. An attacker sends an unsolicited ARP reply saying, “192.168.1.1 is at 00:AA:BB:CC:DD:EE” (the attacker’s MAC). The victim updates its ARP cache with this false mapping. Now, all traffic meant for the gateway is sent to the attacker, who can inspect, modify, or forward the data—a classic man-in-the-middle attack.

Why ARP spoofing works: ARP is a stateless protocol. Devices accept ARP replies even if they didn’t send a request. There’s no authentication or validation built into ARP.

How to prevent ARP spoofing:

• Static ARP entries: Manually define IP-MAC mappings, but this isn’t scalable for large networks.

• Dynamic ARP Inspection (DAI): A network switch feature that checks ARP packets against trusted sources (like DHCP snooping tables) and discards invalid entries.

• Packet filtering: Intrusion detection systems can monitor for suspicious ARP traffic.

• Encryption: Use end-to-end encryption (e.g., HTTPS, VPNs) to mitigate data exposure even if traffic is intercepted.

ARP spoofing remains a common LAN attack vector, especially in public Wi-Fi environments. Preventing it requires combining network design best practices with security-focused configurations.

What is an ARP table and why is it important?

An ARP table—short for Address Resolution Protocol table—is a local cache maintained by a device on a network that maps IP addresses to their corresponding MAC (Media Access Control) addresses. It’s essentially a short-term memory bank that your device uses to keep track of “who is who” on the local network. Without this table, every IP-based interaction on a local area network (LAN) would require a fresh ARP request, leading to slower communication and unnecessary network noise.

Here’s how it works in context: when your device wants to send data to another machine on the same LAN, it needs the MAC address of that device to construct an Ethernet frame. If it doesn’t already know the MAC address, it sends out an ARP request (a broadcast asking “Who has this IP address?”), and the target device responds with its MAC address. Your device then records this information in the ARP table for future use. The next time you need to reach that same IP, your device can skip the broadcast step and retrieve the MAC address from the ARP table directly—saving time and bandwidth.

Each entry in the ARP table includes:

• The IP address

• The corresponding MAC address

• The type of entry (dynamic or static)

• The network interface associated with the entry

Dynamic entries are automatically learned and typically expire after a few minutes of inactivity. Static entries are manually configured by an administrator and don’t expire, making them useful for securing critical devices or preventing spoofing attacks.

The ARP table plays a critical role in network efficiency and stability:

• It reduces broadcast traffic by avoiding repeated ARP requests.

• It improves performance by caching known mappings.

• It enables troubleshooting, as checking the ARP table can help identify misconfigured devices, IP conflicts, or even suspicious behavior like ARP spoofing.

For example, if your device is suddenly sending data to the wrong destination, reviewing the ARP table might reveal that the MAC address associated with a familiar IP has changed—potentially indicating a man-in-the-middle attack.

To inspect the ARP table:

• On Windows, use arp -a

• On Linux/macOS, use ip neigh show or arp -a

• On Cisco devices, use show ip arp

You can also clear or flush the ARP table if needed (e.g., after a network change) using appropriate commands for your operating system.

In short, the ARP table may be small and often overlooked, but it’s a linchpin of smooth local network communication. Whether you’re a network engineer or just someone troubleshooting connectivity issues, knowing how the ARP table works—and how to interact with it—gives you valuable insight into the inner workings of your LAN.