Static vs dynamic IP addresses

The distinction applies twice: to the public address your ISP gives your router, and to the private addresses your router gives each device. You can have a dynamic public IP and still pin your printer to a fixed private one.

Dynamic addressing runs on DHCP, the Dynamic Host Configuration Protocol. When a device joins a network it has no address, so it broadcasts a request to anyone listening. The DHCP server, which is your router at home and your ISP's equipment for the router's public side, answers with an address, the subnet mask, the gateway and the DNS servers to use, all bundled in one exchange.

The address comes with a lease time: a few hours on public Wi-Fi, often a day on home routers, anything from hours to weeks on ISP networks. Halfway through the lease the device asks to renew, and the server almost always says yes to the same address. The address only goes back into the pool if the device is silent past the expiry, which is why a laptop that was away for a week may come back to a different number while one that is always on keeps its address for months.

If your public address changes more often than you expect, one of these is the cause:

• Your router was off longer than the lease. A power cut or an overnight switch-off lets the ISP's lease expire, and the next connection draws a fresh address from the pool.
• Your ISP uses PPPoE or short leases. Some DSL and fiber providers assign a new address at every session, so a modem reconnect means a new IP.
• Your ISP rotates addresses on purpose. A few providers reassign periodically, partly to discourage running servers on residential plans.
• You are on mobile data. Carriers assign addresses per session and move devices between gateways, so the address can change several times a day.
• You are behind carrier-grade NAT. The public address is shared with other customers and chosen by the carrier's NAT gateway, not by your device; it can differ from one connection to the next. The NAT type test tells you whether this applies to you.
• A VPN is on. Every reconnect may land on a different exit server.

Private addresses inside your home change for a simpler reason: the device was away long enough for the router to reassign its address.

There is no flag on the address itself; you check the configuration, or watch what happens.

Public IP (the one websites see)

1. Note the address shown on the homepage IP check or in the IP address lookup.
2. Turn the router off for at least ten minutes, ideally overnight, then check again. A changed address means dynamic. An unchanged one is suggestive but not proof, since the ISP may simply have renewed the same lease.
3. Check your ISP account page or contract. Static public IPs are sold as a named add-on; if you never ordered one, you have a dynamic address. The what is my ISP page identifies the provider if you are not sure whose connection you are on.

Private IP on Windows 11

Open a terminal and run ipconfig /all. Under your adapter, DHCP Enabled . . . : Yes together with Lease Obtained and Lease Expires lines means dynamic. DHCP Enabled . . . : No means a static address was typed in. The same is visible in Settings > Network & internet > Wi-Fi (or Ethernet) > your connection > IP assignment, which reads Automatic (DHCP) or Manual.

Private IP on macOS

Open System Settings > Network, select Wi-Fi or Ethernet, click Details… and open TCP/IP. Configure IPv4 reads Using DHCP for dynamic or Manually for static; a Renew DHCP Lease button appears only when DHCP is in use.

Phones

Android shows IP settings: DHCP or Static in the Wi-Fi network's details; on iPhone, the network's i page shows Configure IP as Automatic or Manual. Mobile-data addresses are always dynamic.

• Hosting something people connect to: a game server, a website, a Plex or Jellyfin server for friends, a camera you view from outside. With a dynamic address, the one you shared stops working after a change.
• Remote access to your own network: a VPN server at home, remote desktop, SSH to a NAS. Same problem: you need to know the address from the other end.
• Allow-lists: a workplace firewall, a database host or a payment gateway that only accepts connections from approved addresses. They cannot approve an address that keeps changing.
• Sending mail from your own server: reverse DNS and sender reputation attach to a fixed address; residential dynamic ranges are widely blocked.

For the first two, a cheaper alternative is dynamic DNS: a small client on your router or NAS updates a hostname such as home.example.net whenever your address changes, so you connect by name and never need the number. Most routers have a DDNS section built in. It does not help with allow-lists, which want a fixed address, not a name. Before opening anything to the internet, confirm with the port checker that the port actually answers from outside; if it does not, carrier-grade NAT, not your address type, is usually the obstacle.

These solve different problems and are often confused.

• A DHCP reservation is set on your own router: it ties a device's MAC address to a fixed private address, for example your printer always gets 192.168.1.50. It is free, takes a minute, and is the right fix for printers, NAS boxes, cameras and anything you port-forward to. The device still uses DHCP; it just always receives the same answer.
• An ISP static IP fixes your public address. It is a paid add-on on most residential plans and standard on business plans, and it is the only way to get a fixed address that the outside world can use. Ordering one often also moves you off carrier-grade NAT, which is sometimes the real reason people want it.

If your goal is reaching a device at home from outside, you typically need both: a reservation so the device's private address never moves, a port forward or VPN pointing at it, and either a static public IP or dynamic DNS so you can find the house. Our guide on changing your IP address covers the manual settings on each platform, and the local IP finder shows the private address you would be reserving.