The International Mobile Subscriber Identity, abbreviated as IMSI, is the internationally standardised unique number to identify a mobile subscriber, as defined in ITU-T Recommendation E.212.
The IMSI consists of:
- a Mobile Country Code (MCC) identifying the country;
- a Mobile Network Code (MNC) identifying the mobile network operator (MNO);
- a Mobile Station Identification Number (MSIN) identifying a device (via the SIM card).
When dealing with devices in a single country, the MCC can be dropped out and the combination of the MNC and the MSIN, which is a 9-digit code identifying the subscriber, is usually referred to as the National Mobile Subscriber Identity (NMSI).
As long as a subscriber has not changed his SIM card, the IMSI will remain the same even when switching mobile device.