arp -aList the ARP table: local devices your Mac knows, with IP and MAC addresses.
When to use it
Use arp -a to find a device's IP address on your network (a printer, a Raspberry Pi), to see which devices are around, or to check for duplicate IP addresses. It only lists devices your Mac has recently communicated with.
Steps
Show the table
Each line is one device your Mac has exchanged packets with recently.arp -aFill the table first
To discover more devices, ping the broadcast address of your network (for 192.168.1.0/24 it's 192.168.1.255), then run arp -a again. Not every device answers.ping -c 3 192.168.1.255Find a specific device
The first six hex digits (the OUI) identify the manufacturer; search them in the IEEE's public OUI list.Remove a stale entry
If a device changed its address, delete the old entry. macOS relearns it on the next packet.sudo arp -d 192.168.1.50
Output explained
router.lan (192.168.1.1) at 72:1d:18:77:9c:11 on en0 ifscope [ethernet]
iphone (192.168.1.2) at b6:47:a4:5d:51:c9 on en0 ifscope [ethernet]
? (192.168.1.6) at (incomplete) on en0 ifscope [ethernet]
- The name comes from local name resolution (mDNS or the router's DNS);
?means none was found. - The IP and MAC address pair is what ARP maps.
(incomplete)means your Mac asked for that address and got no answer: the device is offline or left.- Many phones and Macs use randomized (locally administered) MAC addresses, recognisable by a second hex digit of 2, 6, A or E, like
b6:and72:above.
Options and variations
| Flag | Meaning | Example |
|---|---|---|
| -a | Show all entries | arp -a |
| -n | Don't resolve names (faster) | arp -an |
| -i IF | Only one interface | arp -a -i en0 |
| -d HOST | Delete an entry (sudo) | sudo arp -d 192.168.1.50 |
| -d -a | Delete all entries (sudo) | sudo arp -d -a |
Common errors
| Error | Cause | Fix |
|---|---|---|
| Device missing from the list | Your Mac hasn't talked to it recently | Ping it or the broadcast address, then run arp -a again. |
| (incomplete) entries | No reply to the ARP request | The device is off or on another network; entries expire on their own. |
Do it in the app
LightAtom's Map separates connections to local devices from internet ones, listing them under Local network.
Frequently asked questions
What does arp -a show?
The ARP cache: local devices your computer has recently exchanged packets with, mapping each IP address to a hardware (MAC) address.
How do I see all devices on my network from a Mac?
arp -a shows recently contacted ones. Ping the broadcast address first to fill the table, or check your router's connected-devices page, which knows every DHCP client.
Why do devices have random MAC addresses?
iPhones, Macs, Android and Windows devices use private (randomized) Wi-Fi addresses per network to prevent tracking. These are locally administered, so they don't map to a manufacturer.