Your IP shifts because of DHCP lease renewal, CGNAT sharing, or a new network path — not because something broke. Here's the mechanism behind each cause.
You check your connection today and the address is different from yesterday — a site logged you out for no reason, a firewall allowlist stopped working, or a check tool simply shows a new number. Nothing on your end changed, so it feels like something is wrong. Almost always, it isn't. An IP address was never meant to be a permanent label; it's a lease, a shared resource, or a routing detail that depends on the path your traffic happens to take, and each of those can change independently of anything you did.
Key Takeaways
- A home IP address is usually leased, not owned — DHCP hands it out for a fixed time and renews it automatically, so it looks stable for months until a renewal or pool reshuffle assigns a different one (RFC 2131).
- On mobile networks and many fixed-line ISPs, your public address is shared with hundreds of other subscribers via Carrier-Grade NAT, and it can rotate between sessions (RFC 6598).
- IPv6's rotating temporary address is a deliberate privacy feature, not a fault — see IPv6 privacy extensions.
- Switching Wi-Fi to cellular, or a VPN/proxy/privacy relay engaging or dropping, changes your path, not your lease — that's a different mechanism with the same visible effect.
- A changing IP is normal and mostly harmless; check what your connection currently reports with IP intelligence rather than assuming a change means something broke.
Mechanism 1: DHCP leases expire and renew
Almost no home internet connection has a permanently assigned public IP. Instead, your router requests an address from your ISP using DHCP (Dynamic Host Configuration Protocol), and the ISP hands one out for a fixed lease time — RFC 2131 defines exactly how that lease/renewal handshake works. Your router doesn't wait for the lease to run out: RFC 2131 has it begin renewing at the halfway mark of the lease (the T1 timer) and, if the original server doesn't answer, start asking any available server at 87.5% (T2). Renewal usually returns the same address, which is why a home connection can look static for weeks or months at a stretch even though nothing about it is permanent.
The address changes when that quiet renewal doesn't happen the way it usually does: the lease expires while your modem is powered off (a storm, an ISP-mandated reboot, an extended trip), or the ISP reclaims and reshuffles its address pool for network-management reasons unrelated to you. Lease duration itself is not standardized — it's a per-ISP configuration choice, so "restart your router and your IP changes" is true for some networks and false for others. There's no universal rule to memorize here; it depends entirely on how your specific ISP has configured its DHCP pool.
Mechanism 2: CGNAT means your address was never yours alone
On mobile data, and increasingly on fixed-line connections too, IPv4 address exhaustion pushed ISPs toward Carrier-Grade NAT (CGNAT): a single public IP address is shared across hundreds or thousands of subscribers at once, using the reserved shared address space defined in RFC 6598 (100.64.0.0/10) internally before translating out to a shared public address. From outside, every one of those subscribers appears to browse from the same IP — and as the carrier's gateway reassigns which internal users map to which public address, the one a website sees for you can shift between sessions, sometimes within the same day.
The practical tell is more specific than the advice you usually see. Every home network already translates addresses, so your laptop's 192.168.x.x LAN address never matches your public IP — comparing those two proves nothing. What actually identifies CGNAT is the address on your router's WAN interface, shown on its status page: on an ordinary connection it equals the public IP a website sees, and under CGNAT it doesn't, typically sitting inside 100.64.0.0/10 because your router is itself behind the carrier's NAT. That mismatch is the design working, not a fault. It also has a side effect worth knowing: because you're sharing a reputation with strangers, one of them behaving badly can get the shared address flagged — see flagged as a VPN when you aren't for how that plays out, and IP geolocation accuracy for why a CGNAT gateway can also put your estimated location somewhere you've never been.
Mechanism 3: IPv6 rotates addresses by design
If your network uses IPv6, the rotation you're seeing may not be your ISP at all — modern operating systems generate a temporary IPv6 source address and cycle it periodically, specifically so a single device can't be re-identified across sessions just by its address. That's the privacy extensions mechanism, standardized in RFC 8981, and it's working as intended, not a sign of an unstable connection. Your ISP-assigned network prefix — the part that indicates which network you're on — typically stays put underneath the rotating suffix, so the change is narrower than it looks. IPv6 privacy extensions covers exactly how that rotation works and what stays constant.
Mechanism 4: you changed paths, not addresses
Sometimes nothing about your lease or your carrier's NAT pool changed — you changed which network your traffic takes. Switching from Wi-Fi to cellular data hands you an entirely different ISP's address space. A VPN or corporate proxy connecting mid-session substitutes its own exit address for yours, and disconnecting hands your original one back. A browser-level privacy relay does the same thing at a narrower scope. None of this is a malfunction — it's the expected result of traffic taking a different exit point — but it produces the identical symptom as a lease or CGNAT change: a website suddenly sees a new IP. Privacy tools comparison walks through how VPNs, proxies, and Tor differ in exactly this respect, and Chrome's IP Protection — announced for retirement in October 2025, but still the clearest illustration of the idea — was a far narrower version of the same move, rerouting only third-party tracker requests in Incognito.
What you actually feel when your IP changes
The mechanism is invisible; the symptoms are not. A changed IP can log you out of a session that was tied to your old address, break a firewall or allowlist rule configured for a specific IP, flip which region's content or pricing a geo-aware site shows you, and — the one that stings — land you on a previously abused address and get you challenged as suspicious through no fault of your own. That last case is common enough to have its own detailed breakdown in flagged as a VPN when you aren't: most of these false flags trace back to exactly the shared-address and stale-data effects described above.
What you can check right now
You don't have to guess which mechanism is responsible. BrowserInsight's IP intelligence tool shows your current address, the organization/ASN it belongs to, and its estimated geolocation. Load it, note the reading, and compare it again later:
- If the address changed but the ASN/organization and rough location stayed the same, that's almost certainly a DHCP lease renewal or a CGNAT reassignment — the same provider, a different address from its pool.
- If the ASN or organization changed entirely, you switched networks or paths — Wi-Fi to cellular, or a VPN/proxy engaging or disengaging.
- If only the low-order part of an IPv6 address changed while the prefix stayed constant, that's IPv6 privacy extensions doing exactly what they're supposed to.
An honest closing thought
A changing IP address is the normal, expected behavior of how internet addressing actually works — leases expire, NAT pools reshuffle, networks get switched, and IPv6 rotates on purpose. None of that is a problem to fix. If anything, the more notable case is the opposite one: an address that never changes for months is more identifying, not less, because it gives every site you visit a stable handle to correlate your visits by. This isn't a pitch to go buy a static IP or a particular VPN service — it's simply worth knowing that stability and privacy pull in different directions here, and the "problem" you came here worried about is usually the healthier default.
Frequently Asked Questions
Why did my IP address change overnight?
Most likely a DHCP lease renewal handed you a different address from your ISP's pool, or — if you're on mobile or a CGNAT connection — the carrier's shared-address gateway reassigned which public IP your session maps to. Neither indicates a problem with your connection.
Does restarting my router change my IP address?
Sometimes. If your lease had already expired or was close to it, a restart can trigger a fresh DHCP request that returns a different address. If the lease is still active, many ISPs will simply hand back the same one. It depends on your specific ISP's DHCP configuration, not a universal rule.
Why is my mobile IP different almost every time I check?
Mobile carriers route large numbers of subscribers through Carrier-Grade NAT, sharing a small pool of public addresses across many devices. Which public address your session gets mapped to can change between connections, sometimes within the same day, independent of anything you did.
Is a changing IP address a security risk?
No — on its own, it's normal network behavior, not a vulnerability. The more relevant question is what your current address exposes, not how often it changes; check that with an IP intelligence lookup rather than worrying about the change itself.
Conclusion
An IP address changes for one of four reasons: a DHCP lease renewed onto a new address, a CGNAT gateway reshuffled which shared address maps to you, IPv6 privacy extensions rotated your temporary suffix on schedule, or you switched which network or exit point your traffic takes. All four are the system working as designed. If you want to know what a change actually means for your specific connection, the fastest path is to look at what it reports right now.
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