An International Mobile Subscriber Identity or IMSI ( /ˈɪmziː/) is a unique identification associated with all GSM, UMTS and LTE network SIM cards. It is stored as a 64 bit field and is sent by the phone to the network. It is also used for acquiring other details of the mobile in the Home Location Register (HLR) or as locally copied in the Visitor Location Register. To prevent eavesdroppers identifying and tracking the subscriber on the radio interface, the IMSI is sent as rarely as possible and a randomly-generated TMSI is sent instead.
The IMSI is used in any mobile network that interconnects with other networks. This number is provisioned in the phone directly or in the R-UIM card (a CDMA analogue equivalent to a SIM card in GSM).
An IMSI is usually presented as a 15 digit long number, but can be shorter. For example MTN South Africa's old IMSIs that are still being used in the market are shown as 14 digits. The first 3 digits are the Mobile Country Code (MCC), and is followed by the Mobile Network Code (MNC), either 2 digits (European standard) or 3 digits (North American standard). The length of the MNC depends on the value of the MCC. The remaining digits are the Mobile Subscription Identification Number (MSIN) within the network's customer base.
The IMSI conforms to the ITU E.212 numbering standard.
Read more about International Mobile Subscriber Identity: Examples of IMSI Numeric Presentation, IMSI Analysis, Home Network Identity
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