IMEI Glossary – technical terms explained

The vocabulary around device verification, in plain language.

IMEI (International Mobile Equipment Identity)

A unique 15-digit number identifying every GSM device on a mobile network. It stays with the hardware, which is what makes it useful for establishing a device's history.

TAC (Type Allocation Code)

The first eight digits of an IMEI. They identify the model and the manufacturer, which is how a lookup can name the exact device before any database is queried.

Luhn algorithm (check digit verification)

The formula behind the last digit of an IMEI. It catches typing errors and transposed digits, so an IMEI that fails the Luhn test was mistyped rather than faked.

Check digit

The fifteenth digit of an IMEI, derived from the preceding fourteen by the Luhn algorithm.

GSMA (GSM Association)

The industry body of mobile network operators. It maintains the shared device registries that make a blacklist entry effective across borders rather than only on one network.

IMEI blacklist

The barring list of devices reported lost, stolen or otherwise blocked. A listed device loses access to mobile networks, in many cases internationally.

GSX (Global Service Exchange)

Apple's service platform for authorised partners. It is the source behind detailed Apple device reports: purchase date, warranty, AppleCare, repair and replacement history.

SIM lock (network lock / carrier lock)

A restriction set by an operator that binds a handset to its network. Contract-subsidised devices are frequently locked, and it is independent of any blacklist entry.

Activation Lock / Find My

Apple's binding of a device to an Apple Account. While active, the device cannot be set up after a reset without the owner's password. Only the registered owner can release it.

MDM (Mobile Device Management)

Enterprise device management. A device enrolled in an organisation's MDM will re-enrol itself after a reset and cannot be used freely — a common and expensive surprise on ex-corporate stock.

Knox warranty bit

A one-way counter in Samsung devices that trips when unsigned firmware is flashed. Once tripped, Samsung Pay, Secure Folder and Knox enterprise features stay disabled permanently, and it cannot be reset.

IMEISV

An IMEI extended by a software version field. It identifies not just the device but the firmware level it reports to the network.

MEID

The identifier used by CDMA devices, largely superseded by the IMEI but still present on older hardware.

CEIR (Central Equipment Identity Register)

A central register of device identities. National registers use it to exchange barring information, which is what turns a report filed in one country into a bar in another.

eSIM and EID

An eSIM is a SIM profile provisioned in software rather than on a card. The EID identifies the embedded chip itself and is separate from the IMEI.

IMEI checks in detail

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