What does ARP stand for?

ARP stands for Address Resolution Protocol.

No. DNS maps names to IP addresses. ARP maps IPv4 addresses to MAC addresses on the local network.

Does ARP work over the internet?

No. ARP is local-link only. Routers do not forward ARP broadcasts across the internet.

No. IPv6 uses Neighbor Discovery Protocol, or NDP, for the same local-link role.

What is an ARP table?

An ARP table is a local cache of IP-to-MAC mappings the device has recently learned.

Why should DNS operators care about ARP?

Because many 'DNS is broken' reports are actually local network reachability problems. If DNS returns the right IP but the device cannot reach the local gateway, ARP or local routing may be the real issue.

Fundamentals

ARP Explained - How IP Addresses Reach Local Devices

What ARP does, how IPv4 devices map IP addresses to MAC addresses on a local network, and how ARP differs from DNS.

Updated 3 June 2026

TL;DR

ARP, the Address Resolution Protocol, maps an IPv4 address to a MAC address on the local network. DNS answers 'what IP address belongs to this name?' ARP answers 'what local Ethernet address owns this IP on this link?' ARP is not used across the whole internet and it is not used by IPv6, which has Neighbor Discovery instead.

What you'll learn

Define ARP in plain language
Explain how ARP fits after DNS resolution
Distinguish IP addresses, MAC addresses, and hostnames
Recognize common ARP troubleshooting symptoms

ARP is the last-mile lookup that happens on an IPv4 local network.

DNS can tell your computer:

printer.office.example -> 192.168.1.50

But Ethernet still needs a MAC address to deliver the frame on the local network:

192.168.1.50 -> 8c:16:45:4a:21:10

That second lookup is ARP.

DNS vs ARP

The clean split:

Question	Protocol
What IP address belongs to this name?	DNS
What MAC address owns this IPv4 address on my local link?	ARP

A typical web request goes like this:

Browser needs www.example.com.
DNS resolves it to an IP address.
The operating system decides where to send the packet.
If the next hop is on the local network, ARP finds the next hop's MAC address.
The Ethernet frame is sent.

For internet destinations, ARP usually finds your router's MAC address, not the website's server.

How ARP Works

Suppose your laptop is 192.168.1.20 and your router is 192.168.1.1.

The laptop wants to send a packet to the router but does not know the router's MAC address.

It broadcasts:

Who has 192.168.1.1? Tell 192.168.1.20.

The router replies:

192.168.1.1 is at 00:11:22:33:44:55.

The laptop stores that mapping in its ARP table for a short time.

ARP Is Local Only

ARP broadcasts stay on the local network segment.

They do not cross routers.

That is why your laptop does not ARP for 93.184.216.34 when visiting a public website. It ARPs for the local gateway, sends the packet to the gateway, and the gateway routes it onward.

laptop -> ARP for router MAC -> router -> internet

ARP Tables

Operating systems cache ARP results.

Examples:

arp -a
ip neigh

Typical output:

192.168.1.1 at 00:11:22:33:44:55 on en0
192.168.1.50 at 8c:16:45:4a:21:10 on en0

Entries expire because devices move, reboot, sleep, or change network interfaces.

ARP and IPv6

IPv6 does not use ARP.

It uses Neighbor Discovery Protocol (NDP), which runs over ICMPv6. The job is similar: discover local neighbors and resolve link-layer addresses. The mechanism is different.

If you are debugging IPv6 reachability, ARP commands are the wrong tool. Look for neighbor-cache commands instead:

ip -6 neigh

Common ARP Problems

Duplicate IP addresses

Two devices claim the same IPv4 address. Traffic becomes intermittent or goes to the wrong device.

Symptoms:

one device disappears from the network
pings work, then fail
ARP table flips between two MAC addresses

Stale ARP cache

A device changed MAC address, moved networks, or was replaced. Another device still has an old mapping cached.

Flush the ARP cache or wait for it to expire.

Wrong VLAN or subnet

ARP cannot find devices that are not on the same local link.

If two devices are on different VLANs, traffic needs routing. ARP alone cannot cross that boundary.

Local gateway unreachable

DNS can be perfect and the internet can still fail if the machine cannot reach its gateway.

That is why DNS troubleshooting should separate name resolution from network reachability.

Quick Troubleshooting Flow

If a hostname does not load:

Check DNS:

dig +short www.example.com

Check routing:

ip route get 93.184.216.34

Check local neighbor state:

arp -a
ip neigh

Ping the gateway:

ping 192.168.1.1

If DNS returns the expected IP but the gateway is unreachable, you are no longer debugging DNS.

Frequently asked questions

What does ARP stand for?: ARP stands for Address Resolution Protocol.
Is ARP part of DNS?: No. DNS maps names to IP addresses. ARP maps IPv4 addresses to MAC addresses on the local network.
Does ARP work over the internet?: No. ARP is local-link only. Routers do not forward ARP broadcasts across the internet.
Does IPv6 use ARP?: No. IPv6 uses Neighbor Discovery Protocol, or NDP, for the same local-link role.
What is an ARP table?: An ARP table is a local cache of IP-to-MAC mappings the device has recently learned.
Why should DNS operators care about ARP?: Because many 'DNS is broken' reports are actually local network reachability problems. If DNS returns the right IP but the device cannot reach the local gateway, ARP or local routing may be the real issue.

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