How a SIM Gets Its IP Address, and Why It Changes

People renting a mobile proxy for the first time often expect it to work like a server: one box, one address, forever. Carrier networks do not work that way, and understanding why saves a lot of confusion later. This article follows the address from the moment our modem registers a SIM on the network to the moment a website sees it, explains what a rotation does under the hood, and describes how often a SimProxies SIM changes address when you leave it alone.

From SIM to gateway

A SIM card does not have an IP address. It has an identity on the carrier network, and when the modem holding it powers on, it attaches to a nearby cell and asks for a data session. The carrier's core network answers by routing that session through a gateway that serves the region, and the gateway gives the modem a private address on the carrier's side.

The public address that websites see belongs to the gateway, not to the SIM. The carrier maps many private addresses onto each public one, a process usually called carrier-grade network address translation. This is why a US carrier address is shared by many real phones at the same time, and why it is trusted: blocking it would block paying subscribers.

Why the address is tied to a metro

Gateways serve regions, and the public addresses they use are registered to those regions in the geolocation databases that websites consult. A SimProxies SIM in Houston attaches to a Texas gateway, so its address resolves to Houston or to Texas. It will not resolve to a street, because nothing in the carrier network knows a street. It may occasionally resolve to a nearby city for a few hours when a database lags, which is normal across every carrier.

The upshot for you is that city choice is the geolocation control you have. Pick the metro that matches the work, and the rest follows from the carrier.

What a rotation does

When you rotate a SimProxies line, the modem drops its data session and opens a new one. The gateway assigns the new session a private address, and the public address it maps to is drawn from the same regional pool. Sometimes it is a different public address; sometimes, because pools are finite, it is one the SIM has used before. Either way it is still the carrier, still the city, and still an address shared with ordinary phones.

You can trigger a rotation from the dashboard, from the rotation link, from the API, on a timer, or by hand. There is no limit on how many times you do it, and nobody else can do it on your SIM. The one thing to expect is a short pause while the modem reconnects, typically a few seconds, during which open connections will drop.

Why the address changes when you did not ask

Left alone, a dedicated SIM holds its address for as long as the carrier keeps the data session alive. Carriers do end sessions on their own: a tower handover in a busy area, a maintenance window, a daily reset that some networks apply to long-lived sessions. When that happens the modem reconnects automatically and usually lands on a new address.

On a line you share with nobody, these events are the only unplanned changes you will see, and they are infrequent. On a pooled service the same thing happens plus every rotation any other customer triggers, which is the difference people notice most when they move to a dedicated SIM.

• Planned: you rotate from the dashboard, link, API, timer or by hand
• Unplanned but normal: the carrier ends a session and the modem reconnects
• Never: another customer rotating your line, because there is no other customer

Sticky sessions and how long they last

A sticky session simply means you do not rotate. How long the carrier will leave a session alone varies by network and by city, and we do not promise a figure because the carrier does not publish one. In practice a dedicated SIM that is kept busy holds its address for a long time; the sessions that get reset tend to be the idle ones. If your work needs the address to hold across a long login, keep a little traffic flowing and avoid rotating in the middle.

What this means for how you use the line

Treat the address as stable but not permanent. Record it when a session starts, rotate between jobs rather than during them, and expect a reconnect now and then. Match the device settings to the SIM's city so that an unplanned address change still tells the same story. And if you need a brand-new address right now, rotate and check: the SIM will give you one from the same carrier in the same metro, which is the whole reason you rented it.

Frequently asked