IMSI (International Mobile Subscriber Identity)
An International Mobile Subscriber Identity (IMSI) is a unique number, usually fifteen digits, associated with Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS) network mobile phone users. The IMSI is a unique number identifying a subscriber and is used internally within mobile networks and not made public.
The first 3 digits represent the mobile country code (MCC), which is followed by the mobile network code (MNC), either 2-digit (European standard) or 3-digit (North American standard). The length of the MNC depends on the value of the MCC, and it is recommended that the length is uniform within a MCC area. The remaining digits are the mobile subscription identification number (MSIN) within the network's customer base, usually 9 to 10 digits long, depending on the length of the MNC. [MCC][MNC][MSIN]
A single IMSI is normally associated with a single MSISDN.