Short answer: tell your router to always give that device the same address. This is called a DHCP reservation (some routers call it address reservation or manual assignment). You enter the device’s MAC address and the address you want, inside your router’s normal range, then restart the device so it picks up the new address. It’s simpler and safer than setting a static IP on the device itself. If the device is a phone, set its private Wi-Fi address to Fixed for your network, or the reservation can stop matching.

Why addresses change

Your router hands out local addresses automatically using DHCP. ASUS explains that a device normally gets a dynamic address this way, and that the same device may not get the same address each time it connects. That’s fine for a phone or laptop, but not for a printer other devices need to find, a NAS, a camera, or anything you’ve set up port forwarding for: if its address changes, the rule points at nothing.

Reservation or static IP?

There are two ways to fix an address:

  • A reservation in the router (recommended). The router keeps handing out addresses as usual but always gives this device the same one.
  • A static IP set on the device. The device ignores the router and uses an address you typed in. It’s easy to pick an address that clashes with another device.

ASUS recommends leaving the device set to obtain its address automatically, and says that if you do set a static address on the device, it has to match the router’s reservation.

How to set one up

  1. Find the device’s MAC address. It’s usually on a label, in the device’s network settings, or in the router’s list of connected devices; see who’s on your Wi-Fi shows where.
  2. Log in to the router (how to log in to your router).
  3. Open the reservation settings. On ASUS routers it’s under LAN, DHCP Server, where you enable manual assignment and add the device under Manually Assigned IP; ASUS’s app calls the list the IP Binding List. On NETGEAR routers it’s under Advanced, Setup, LAN Setup, Address Reservation.
  4. Pick the address. Choose one in the router’s own range; NETGEAR’s example is an address of the form 192.168.1.x.
  5. Save.

Other brands use similar names: look for DHCP reservation, address reservation or static lease.

Make it take effect

The device keeps its old address until it next asks the router for one. NETGEAR says the reserved address isn’t assigned until the device next contacts the router’s DHCP server, so restart it or force it to release and renew its address. On Windows, ipconfig /release followed by ipconfig /renew does that. Then check the device’s network settings to confirm the new address.

Phones and private Wi-Fi addresses

Reservations match on the MAC address, and modern phones don’t always use their real one. Apple devices use a different private Wi-Fi address for each network, with three settings per network: Off, Fixed or Rotating. A Rotating address changes every two weeks, and even a fixed one changes if you reset network settings or the device stays away from the network for six weeks.

Fixed addresses matter most for printers, NAS boxes, cameras, game consoles and anything you’ve opened ports for; for console gaming, NAT types explained covers why.