An IMSI (International Mobile Subscriber Identity) is a unique code used to identify an individual user on a GSM or UMTS network.  The IMSI is a 64-bit number which is usually represented as 15  decimal digits.

An IMSI consists of three components:

  • Mobile Country Code (MCC) (3 digits)
  • Mobile Network Code (MNC) (2-3 digits)
  • Mobile Subscriber Identity Number (MSIN)

The Mobile Country Codes and Mobile Network Codes are listed at MCCList.

In GSM networks, the IMSI is stored in the Subscriber Identity Module (SIM).  In CDMA networks, it is stored in the R-UIM card.  In other networks, the IMSI is stored in the phone handset itself.

The IMSI is used to generate the TIMSI.  The TIMSI is usually transmitted over-the-air in place of the IMSI in order to protect the IMSI from eavesdropping.