IMSItunnuksen

IMSI-tunnus, or International Mobile Subscriber Identity (IMSI), is a unique identifier assigned to a mobile subscriber. It is used by mobile networks to identify and authenticate the subscriber to the home network and to support roaming. The IMSI is stored on the subscriber’s SIM or eSIM and is linked to the subscriber’s secret key Ki, which is used in the authentication process.

Structure and storage: A typical IMSI is up to 15 digits long and consists of three parts: the Mobile Country Code (MCC, three digits), the Mobile Network Code (MNC, two or three digits), and the Mobile Subscription Identification Number (MSIN, up to ten digits). The exact length of the MSIN varies by operator and country. The IMSI is kept on the SIM card and is not meant to be exposed unnecessarily.

Use in networks: When a device connects to a mobile network, the network uses the IMSI to locate and verify the subscriber’s profile in the home network. In practice, networks often avoid transmitting the IMSI over the air and instead use temporary identifiers, such as TMSI (in older generations) or GUTI (in LTE). In 5G, the permanent identity is SUPI, which is derived from the IMSI and transmitted in encrypted form as SUCI to protect privacy.

Security and privacy: The IMSI is a sensitive personal identifier. Unauthorized disclosure can enable fraud or subscriber tracking. Access is restricted to trusted network components, and operators implement privacy safeguards to minimize exposure, especially during roaming and initial network attach.