How Big Is the IP Pool Behind One Dedicated Modem?

People ask for a number: how many addresses does one modem give me? The honest answer is that it depends on the carrier's regional pool, on how often you rotate, and on luck, and that you can measure it yourself in an afternoon. This article explains where the addresses come from, what shapes the count over a day and a month, how to log it, and why seeing the same address twice is expected rather than a fault.

Where the addresses come from

A modem does not own an address. When it registers and opens a data session, the carrier's packet gateway for that region assigns one from the pool it manages. Each carrier runs a number of these gateways across the country, and each serves a wide area, often more than one state. When the session ends, by rotation or by the carrier's own timer, the address goes back to the pool and the next session gets whichever one is free.

That means the pool your modem draws from is the regional pool for the gateway it lands on, not a pool belonging to the city or to us. It is large, it is shared with the carrier's real subscribers, and it is the same pool whether the modem is shared or dedicated.

What limits the count

Three things. The pool's size, which is the carrier's business and which differs by carrier and region. The gateway's allocation habit: some gateways hand out addresses from a short list for a while before moving on, so a modem that rotates quickly can see the same few addresses in a loop. And how often you rotate, because an address that never changes is a count of one, however large the pool.

There is also a subtlety about session resets. If you rotate within seconds of the previous rotation, the gateway may return the address you just had, because it was the most recently freed. Spacing rotations out by a minute or more tends to yield more variety, though nobody can promise a fresh address every time.

Over a day and over a month

If you leave the modem alone, you will usually keep one address for hours or days, until the carrier resets the session. If you rotate a handful of times a day, you will see a handful of addresses, with occasional repeats, most of them in the same subnet ranges. Over a month of steady rotation the set of distinct addresses grows, but it grows more slowly than the number of rotations, because repeats become more frequent as you have seen more of the gateway's current list.

That is the general shape. The actual numbers are specific to your carrier, your city and the month, which is why the only useful figure is the one you measure.

How to measure your own pool

Write a short loop. Call the rotation link from the dashboard, wait for the modem to come back, fetch an address check page through the line, and append the address with a timestamp to a file. Run it for a day at an interval that matches how you intend to use the line. Then count distinct addresses, count repeats, and look at the subnet prefixes. Do the same over a week if you need a longer view.

The result tells you what this modem on this carrier in this city does, which is the only thing that matters for your work. If the variety is not enough, the practical fix is more modems, in more cities or on more carriers, not harder rotation of one.

Why repeats are normal and usually fine

An address that comes back is still a carrier address shared with real phones. For most work the repeat is harmless: a platform sees the same mobile address it saw last week, which is what phones do. The cases where repeats matter are narrow, such as a site that rate-limits by address and that you hit hard, and those cases are better served by spreading the load across lines.

What a dedicated modem gives you on top of the pool is control of the timing. Nobody else on the modem means every rotation is yours, so the sequence of addresses in your log is exactly the sequence your accounts experienced.

Rotation methods and the pool

From the dashboard, by the rotation link, by the API, on a timer, or by hand. All of them do the same thing: end the data session and open a new one. None of them can choose the address. If you need a different region's pool, move the line to another city from the dashboard, free, with your remaining time carried over; the modem there sits on a different gateway with a different pool.

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