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What Is ARP (Address Resolution Protocol)? Basics and ARP Spoofing Risks

Walk through, step by step, how an IP address gets translated into a MAC address.

What ARP (Address Resolution Protocol) is

ARP (Address Resolution Protocol) is the protocol that translates an IP address into its actual hardware address, the MAC address, within a local network (LAN). It is one of the oldest and most fundamental protocols underlying internet communication.

Why ARP is needed at all

Data is actually delivered as Ethernet frames addressed by MAC address to devices on the same local network. When you only know a device's IP address, you first need to figure out the MAC address belonging to that IP before you can start communicating β€” and that lookup is exactly what ARP handles.

How ARP requests and replies work

A device broadcasts an ARP request β€” essentially "who has this IP address?" β€” to the entire local network. Only the device that actually holds that IP address responds directly (unicast) back to the requester with its own MAC address.

How to check your ARP table (cache)

Once an IP-to-MAC mapping has been looked up, it is stored in the ARP table (cache) for a period of time so the device does not need to ask again on every packet. On Windows, Mac, and Linux alike, running "arp -a" in a terminal or command prompt shows the current ARP table.

What ARP spoofing (cache poisoning) is

ARP spoofing is an attack where an attacker floods the network with forged ARP replies, tricking other devices into believing that a specific IP address β€” usually the router's gateway address β€” belongs to the attacker's own MAC address. Once that happens, traffic other devices intended to send to the internet instead routes through the attacker first, which can be exploited for a man-in-the-middle attack.

Practical ways to protect against it

Avoiding untrusted public Wi-Fi networks and using a VPN for anything sensitive are the most practical everyday precautions. In business or server environments, static ARP entries that hard-code the gateway's MAC address, along with network monitoring tools that detect and flag duplicate MAC addresses, are commonly used together.

Curious about the MAC address itself?

If you want to know exactly what the MAC address ARP resolves actually is and how it is structured, it is worth reading up on MAC address basics alongside this guide.

It is also related to IP address conflicts

When two devices on the same network end up with overlapping IP addresses, ARP replies can get scrambled and cause connection instability. It is worth checking a guide on resolving IP address conflicts as well.

Frequently Asked Questions

Does ARP get used across the internet, reaching servers in other countries too?

No. ARP only operates within a single local network (LAN). The moment traffic crosses a router into a different network, routing protocols and the IP addressing system take over determining the path, and ARP is repeatedly used only within each individual local segment along the way.

Should an everyday user actually worry about ARP spoofing?

The risk is low on a typical home network, but it is a real, practical attack on public Wi-Fi networks shared with strangers, like at a cafΓ© or airport. It is safest to avoid sensitive activity like logging in or making payments on public Wi-Fi, and to use a VPN alongside it.