International Mobile Subscriber Identity

An IMSI identifies a mobile subscription within the global cellular numbering framework. It contains a three-digit mobile country code (MCC), a two- or three-digit mobile network code (MNC), and a mobile subscriber identification number (MSIN); the complete identifier is limited to 15 decimal digits.¹ The MCC and MNC together identify a Public Land Mobile Network, while the MSIN distinguishes the subscriber within that network.

The IMSI is not the same as a telephone number. A telephone number is generally represented by an MSISDN and may change independently of the subscription identity. Nor is it the same as an IMEI, which identifies a handset rather than a subscriber. Network databases use the IMSI to locate subscription records and invoke services such as authentication, authorization, and roaming.

National administrations and operators receive numbering resources under the ITU-T E.212 framework, while technical details are maintained in 3GPP specifications.²

The IMSI enables a visited network to recognize a subscriber and obtain authorization from the home network during international roaming. In older and current mobile architectures, it is associated with subscriber databases such as the HLR or HSS; in 5G, related subscription data is handled by the Unified Data Management function.

A SIM or USIM stores the long-term identity and cryptographic credentials, but the handset does not normally transmit the IMSI in every connection. After registration, networks generally assign temporary identifiers such as a TMSI in GSM and LTE or a 5G-GUTI in 5G. These temporary names reduce routine exposure of the permanent identity while allowing the network to address the device efficiently.¹

Authentication commonly uses a challenge-and-response procedure known as Authentication and Key Agreement (AKA). The home network verifies the subscription and supplies authentication material; successful authentication also establishes keys used to protect subsequent radio communication.³

Protecting the IMSI matters because possession of the identifier can help correlate a person’s mobile activity across locations and networks. Legacy procedures may request the permanent identity when a temporary identifier is unavailable or rejected, creating an opportunity for a rogue base station or IMSI catcher to solicit it. The risk is greater when older radio technologies or forced fallback are involved.

LTE and earlier systems mitigate exposure mainly through temporary identities, but they do not eliminate every identity-request weakness. 5G introduces a stronger privacy design: the permanent subscription identifier is represented as a SUPI, and the home-network portion can be concealed in a SUCI using public-key protection before transmission over the access network.⁴ This limits the usefulness of passive collection and makes unauthorized recovery more difficult, although implementation, downgrade behavior, and operator configuration still affect practical protection.

Users cannot normally change an IMSI like a password. Replacing it generally requires a new subscription credential or a controlled operator procedure.

The IMSI is a subscription identity, so one physical phone can use different IMSIs when its SIM or eSIM profile changes, and one subscriber can hold several IMSIs across separate subscriptions. Conversely, a single device may contain multiple active profiles, each with its own identity. This distinction is central to dual-SIM operation, enterprise mobility, and machine-to-machine services.

The final MSIN does not necessarily encode a human-readable account number. Operators may allocate it from internal ranges, and the structure can differ between networks even when the same MCC and MNC are used. An MNC can also have either two or three digits; software must preserve the operator’s assigned format rather than infer it from a fixed-length assumption.²

In 5G specifications, IMSI remains relevant because it is one possible form of the Subscription Permanent Identifier (SUPI), despite the newer privacy-preserving SUCI representation. IMSI therefore continues to connect legacy authentication, roaming, and subscriber databases with newer 5G identity procedures.⁴

MCC

Mobile country code: the three-digit component identifying a country or geographic area in the E.212 numbering plan.

MNC

Mobile network code: the two- or three-digit component identifying a mobile network within an MCC.

MSIN

Mobile subscriber identification number: the portion of an IMSI that distinguishes a subscription within its network.

SUCI

Subscription Concealed Identifier: a 5G representation that conceals the permanent subscription identity from the serving network.