IP Location Across Several Lines: Why Two Cities Can Read the Same

A team with lines in Boston and New York runs a lookup on both and sees New York twice. Is one of the lines in the wrong rack? Almost always not. The address a website sees is assigned by a carrier gateway, and the location a website reads is whatever an IP database recorded for that address, which is a description of the gateway's region more than of the device. This article explains how the two can diverge, why some carriers show it more than others, and how a team should check each line.

Two different things called location

The physical location is where the device sits: a modem in our Boston rack, for example. The IP location is what a geolocation database says about the public address that device currently holds. The database does not know where the device is. It knows which carrier owns the address block, and it has made an estimate of the region the block serves, from carrier registrations, measurements and user reports. When the estimate matches the rack, the two agree. When the carrier serves the block from a gateway elsewhere, they do not.

Websites use the IP location. So for your purposes, the IP location is the one that matters, and the physical location explains why it is what it is.

How a carrier gateway sets the address

A mobile device attaches to the carrier's core network and is assigned a public address from a block served by a gateway. Carriers do not keep a gateway in every city. They aggregate traffic from a region through a smaller number of gateways, and the address blocks are registered to those gateways' regions. A device in one city can therefore hold an address whose block is registered to a gateway a few hundred miles away, because that is where its traffic actually enters the public internet.

This is a design choice by the carrier, and it is why the three carriers behave differently. T-Mobile tends to show the metro itself most consistently. Verizon addresses can resolve to a regional hub within the region, so a line in one city may read as another city in the same part of the country. AT&T is in between, with one famous case.

The Boston example

Our Boston rack holds AT&T and Verizon lines. Verizon lines in Boston geolocate as Boston. AT&T lines in Boston geolocate as New York, because AT&T's New England traffic exits through its New York gateway and the address blocks are registered there. Both lines are physically in the same rack; the IP databases describe them differently because the carriers route them differently. Neither is a fault.

For a team, the lesson is to choose the carrier with the IP location in mind. If a line must read as Boston, buy Verizon in Boston. If reading as New York is fine or wanted, AT&T in Boston works and is still a Boston line in every other sense.

Why databases disagree with each other

There is no single geolocation database. Each vendor maintains its own, updates it on its own schedule, and makes its own estimates for carrier blocks, which are the hardest blocks to place because the devices behind them move. One database may place an address in the metro, another in the regional hub, a third at the state's centroid. Websites use whichever vendor they licensed, so the same line can read as two cities on two websites at the same moment.

State and country are reliable across vendors for all three carriers; city is where they diverge. Plan work around the state when you can, and when a job genuinely needs a city, test it on the website that matters rather than on a checker.

What to check per line

For each line, run the lookup on two or three different IP lookup sites and record what they say. You are checking three things: that the owner is the right carrier, that the state matches the metro, and that the city, where it differs, differs in the expected way for that carrier. A Verizon line reading as a hub city in the same region is expected. An AT&T Boston line reading as New York is expected. A line reading as another state is not, and support should hear about it.

Keep the result in the line's notes. When a colleague later sees New York on a Boston line and panics, the note saves a ticket.

• Owner: AT&T Mobility, T-Mobile USA or Verizon Wireless.
• State: matches the line's metro on every lookup.
• City: may be the metro or the carrier's regional gateway; note which.

Planning a multi-metro fleet around it

If the team's work is state-level (most local search, most pricing, most editions), any carrier in the right metro is fine. If the work is city-level, T-Mobile is the most consistent choice, Verizon is reliable for state and variable for city, and AT&T should be checked per metro with Boston being the known case. A free move lets you relocate a line and re-check without buying another.

After rotation, the new address is from the same carrier in the same metro, so the IP location usually stays the same, though it can shift between the metro and the hub as different blocks are assigned. If a task is sensitive to that, check the lookup after each rotation.

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