Internet networking involves a lot of alchemy, and I must confess that I occasionally drop a newt’s eye (or a newt’s IP) into a cooking pot to troubleshoot my local network. There’s a certain message that macOS displays in a limited number of cases, and that’s baffling because it’s the kind of low-level bubbling that Apple generally takes care of. In this case, your Mac is alerting you to a problem that you have caused yourself or that may involve your Wi-Fi gateway or broadband modem. That message: “Another device is using your IP address.”
This conflict prevents your computer from accessing part of the local network and the Internet. This is why.
Every device that communicates over the Internet needs a unique Internet Protocol (IP) address, a number used by routers to package data and send it to the correct recipient. That’s true on a LAN or within top-level internet data exchanges, be it a $10 million router or an addressable smart light bulb. When the internet started its super-fast growth more than two decades ago, the addresses used came from a relatively small range, using the IP version 4 (IPv4) standard. The number of possible unique addresses was much smaller than what people predicted would be needed soon, and that prediction came true.
Network Address Translation (NAT) was created as a way to offer LAN connected devices something special while preserving the available pool of addresses. While most IP addresses must be unique because they are all used in a large public pool, such as having a unique address in a unique city in a unique state or province, the NAT protocol allows private addresses passed through a gateway that maps the private address to a shared public address. Outbound traffic is managed by the router so that incoming responses are routed back to the correct computer or other hardware on the LAN. It’s a tricky process, but it’s used worldwide for trillions (perhaps quadrillions) of data packets per day.
Most routers pair NAT with DHCP (Dynamic Host Configuration Protocol), which automatically assigns addresses to devices when prompted. You will notice that when you connect to a Wi-Fi network or connect to your network (and most networks) via Ethernet, you are not prompted to configure IP settings. Instead, your device is set by default to query the gateway via DHCP; the gateway receives it, the NAT system finds and maintains an available address, and the DHCP server gives that address and other settings to your hardware, which is called a “lease.”
Here are several methods to solve the address-in-use problem.
If you don’t manage your gateway
If a family member, friend or colleague manages the gateway, even if it is as simple as they are the one who has the password and they have never touched it since they set it up, ask them for help and have them read this article.
Router power cycling can help if it’s an error in the router’s internal address tracking. Connecting to the router’s admin interface can also help with troubleshooting.
Sleep and wake up your Mac
If you’ve never touched your gateway settings, you can just try putting your Mac to sleep and waking it up; that sometimes resolves a passing conflict. When the Mac wakes up without an IP address, it tries to re-address the gateway’s DHCP server, and it can work just fine.
You can try restarting your computer, but that step may not be required; try the following solution instead.
Renew DHCP lease
In the macOS Network preference pane, select your network adapter from the list on the left and click Advancedthen TCP/IP. Click on the Renew DHCP lease. If this works, you’re all set (for now). If not, then move on to checking for other issues.
Manually configured address
Each device must have a unique private IP address on the local network, and if you have manually configured your hardware’s network settings to use a specific number, you may see the warning “Another device is using your IP address” because the DHCP/NAT combination has assigned an address that you have set manually for the computer you are on. (Or the other machine using it is configured manually and you or someone else needs to check that.)
For example, you can use a game server or share your screen remotely with your computer, you have read about port mapping or UPnP (Universal Plug ‘n’ Play), and you have configured your machine to use a fixed (or “static”) private address so that it can always be reached through some router magic. For example, you may have set your computer’s address to 192.168.1.100.
Many gateways allow you to reserve specific addresses (sometimes called “DHCP reservations”) to avoid reusing an IP address on the network. Others let you set the start of an IP range. So if the network is 192.168.1.0 to 192.168.1.255, you can set the beginning of NAT mapped addresses to 192.168.1.100 and choose an available address from 192.168.1.2 to 192.168.99. NAT will still work and DHCP is not involved. (The .0 and .1 addresses are usually reserved, so you may need to start at 192.168.1.2 in this example.)
To check if you (or someone else) configured your Mac this way in the past and just forgot, open the Network Preferences pane, select your network adapter from the list on the left, then click Advanced in the lower right corner. In the TCP/IP window, if the Configure IPv4 setting is Manual, the address was entered manually. Check your gateway to see if you can change the range there if you want or need to keep this setting.
The TCP/IP tab allows you to set an address for your Mac, which may lead to choosing an address that is already in use.
If you don’t know why it’s set up this way and it’s not on a work network where a change could affect colleagues, choose Using DHCP in the pop-up menu, click Okayand click Applyand see if the problem goes away.
Not enough network addresses to hand out
Most routers are configured by default to offer anywhere from 100 to 200 addresses because when the box was designed, managing that amount was within the device’s processing capabilities or perceived as a reasonable number. However, an older gateway may be set by default or configured by an ISP’s installer for only 50 dynamically assigned private addresses. In 2000, who could imagine a future where more than 50 different hardware in a house would all have to be connected to the internet?! Ridiculous.
The DHCP server not only assigns an address, but also assigns an expiration time to it. When the time is up, the device can request a new address or refresh the server if the device is currently active on the network. Otherwise, that address will become free and go back into the pool. In some cases, even with hundreds of private addresses available, your gateway may run out of stock. It shouldn’t be handing out an identical address, but it can go wrong. (You may not get an address, in which case your Mac will create a so-called self-assigned IP addresswhich starts with 169.254.xx)
This depletion of numbers can occur if you have a lot of Internet-connected devices, share a house or building with a poorly managed Internet service (because they really should have more addresses available or better DHCP timeouts), or lots of people passing through your network.
Start with your router. Read the manual, log in to the administrative interface and check the settings. It can show you a list of connected devices and the assigned private IP addresses. You can see if you exceed the number it can allocate, and you may be able to increase that number easily. You may also be able to decrease the timeout duration so that addresses are freed up more quickly.
You may need to upgrade your router or make more complicated changes, but that’s unlikely for home and small business use. On most gateways, you should be able to increase the number to more than 200 or make changes that allow you to allocate more than 500 or more than 1,000.
This Mac 911 article answers a question from Macworld reader Humberto.
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