IMSIs

An International Mobile Subscriber Identity (IMSI) is a unique identifier assigned to a mobile subscriber and used by cellular networks to identify and authenticate the subscriber on GSM, UMTS, LTE, and 5G systems. The IMSI is typically stored on the subscriber's SIM or eSIM and is presented to the network during attachment and authentication processes.

An IMSI consists of the Mobile Country Code (MCC), Mobile Network Code (MNC), and Mobile Subscriber Identification Number (MSIN). The MCC is three digits, the MNC is two or three digits, and the MSIN identifies the subscriber within the operator's network. The total length is up to 15 digits.

When a device connects to a network, the network may require the IMSI for initial authentication. After successful authentication, networks typically use temporary identifiers (such as TMSI in GSM or GUTI in LTE/5G) to protect subscriber privacy. The authentication relies on cryptographic keys stored on the SIM and on the home network.

Because the IMSI uniquely identifies a subscriber, it is treated as sensitive personal data. Operators implement measures to minimize exposure, and regulatory frameworks govern its handling. Exposure can occur if a device or a rogue base station forces the IMSI to be transmitted in clear; to mitigate this, the IMSI is rarely sent after initial attach, and encryption is used on signaling. IMSI catchers or "Stingrays" are devices that attempt to force a device to reveal its IMSI.