Duplicate IP Address? How to Check Which Device Really Has That IP

The NAS is at dot twenty. Everyone in the house knows it. Then one day dot twenty opens a different login page, or the backup reaches nothing. An IP address is a handy shortcut for a device, but it isn't the device's identity. In this video we'll check which device really has an address, before you change a setting or type a password into the wrong place.

As always, this is DeviceShelf's demo network, a made-up home with forty-four devices.

Open a device, and you see two addresses. The IP address is what the network uses to route traffic. The MAC address, the hardware address, belongs to the network connection itself. The router hands out IP addresses for a limited time. The hardware address normally stays with the device. So when you want to know which device you're looking at, the hardware address is the better clue.

This NAS has a reservation: the router always gives this hardware address the same IP. That's the setup where dot twenty really stays the NAS.

Most devices get their address on a lease instead, dynamic D H C P. When a phone leaves, its lease runs out, and the router can give that address to the next device that asks. When the phone comes back, it may get a different one. That's normal, and it's the most common reason an old address points somewhere new.

The timeline shows the comings and goings. This guest phone had dot one forty and went offline three hours ago. By tomorrow, dot one forty may belong to a completely different device. Two more causes: a device with both cable and Wi-Fi has two hardware addresses and often two IP addresses, and phones use a private Wi-Fi address that can change. We have a separate video on that.

If you want to know when it happens, switch on this alert under Settings, Alerts. DeviceShelf then tells you when a device that kept the same address for days suddenly moves.

The nastier case is a duplicate address: two devices answer on the same IP at the same time. The symptoms come and go. One device works, then the other, depending on which one answered last. The Network view checks for that after every scan. Here it's clean: no address is answered by more than one device. If there were one, you'd see both hardware addresses side by side.

And right below is the usual cause before it happens. This Raspberry Pi has an address typed in by hand, inside the range the router hands out by itself. It works today. The day the router gives dot ninety-nine to someone else, you have a duplicate. Our video on static IP addresses shows how to fix that.

One more place where an old answer hides: your own computer. It keeps a small table of which hardware address belongs to which IP address, the A R P cache, and it trusts that table for a few minutes. When a device has just swapped addresses with another one, your computer may still talk to the old one for a while. So if a familiar address leads somewhere strange right after a change, wait a few minutes or scan again before you conclude anything. Don't restart the router just to get a quick answer: that throws away the timing that tells you what happened.

So before you change a setting or enter a password at a familiar address, check three things. Does the hardware address match what you expect? Compare it with the router's list or the sticker on the device. Does the device tell you something only it would say, like its model, serial number or host name? And does its certificate name match? A browser warning about a certificate that changed is a reason to stop, not to click through.

For the devices that matter, keep a short note: name, hardware address, IP address and the date you checked. Print label puts the essentials on a sticker for the device itself.

When the records disagree, don't fix three things at once. If the router says dot twenty belongs to one hardware address and your computer says another, note both with the time, and check again after the device has been active for a bit. Delete a reservation, change an address and restart the router all in one go, and you'll never know which step fixed it, or which one broke the next thing.

For a NAS, a printer or a home server, set up a reservation in the router, tied to the hardware address of the connection the device actually uses. Then the address stays put, and the router remains the one place that hands addresses out.

So: an IP address says where, the hardware address says who. Leases move, private addresses change, and old records linger. Check the hardware address before you trust an address, let DeviceShelf watch for duplicates and address changes, and give the important devices a reservation.

More videos about your home network are on our channel. DeviceShelf runs on Mac, Windows and Linux, at deviceshelf dot app.