imsi

IMSI stands for International Mobile Subscriber Identity. It is a unique identifier assigned to a mobile subscriber in cellular networks and stored on the subscriber's SIM or eSIM. The IMSI is used by mobile networks to identify, authenticate, and bill the subscriber when they connect to the network.

Constituent and format: An IMSI consists of three parts: the Mobile Country Code (MCC), the Mobile Network Code (MNC), and the Mobile Subscription Identification Number (MSIN). Together they form usually 15 digits, though lengths may vary by operator and country. The IMSI is kept confidential and is typically transmitted only at initial attachment, with protections applied thereafter.

Usage and privacy: On first access, the device may reveal its IMSI to locate the subscriber's profile in the home location register. After successful authentication, networks usually use temporary identifiers (such as TMSI or GUTI) to protect the subscriber's identity during ongoing sessions.

Security and evolution: The IMSI is a sensitive identity because it can enable tracking. IMSI catchers exploit exposure of IMSI values. To mitigate this, mobile systems employ encryption and temporary identifiers. In 5G, the related Subscription Permanent Identifier (SUPI) and the concealment of SUPI as SUCI improve privacy, while IMSI remains used in legacy contexts.