The International Mobile Subscriber Identity (IMSI) is a network-internal identifier for mobile subscribers. It is located on the SIM card and is used to uniquely identify the network subscriber and for routing within the various mobile communications networks, such as the GSM network, the UMTS network, and Long Term Evolution( LTE).
The 15- digit IMSI number is made up of the Mobile Country Code( MCC), the Mobile Network Code( MNC) and the Mobile Subscriber Identification Number( MSIN). In this constellation, the three-digit Mobile Country Code (MCC) stands for the country code, the two-digit Mobile Network Code (MNC) is the identification number for the mobile network operator, and the ten-digit Mobile Subscriber Identification Number (MSIN) is the subscriber number.
The international mobile subscriber identification (IMSI) code is used to identify each subscriber when switching and signaling a call. Since a subscriber could also be identified by unauthorized persons with the help of the IMSI, a temporary subscriber identifier( TMSI) is used for signaling instead of the IMSI. There is a link between the two numbers, but only the network operator knows it.
The subscriber does not know his IMSI, but only his Mobile Subscriber ISDN Number( MSISDN), via which he can be reached. The reference between the network-internal IMSI and the MSISDN is established by the location register( HLR) and is known only to theHLR. If the SIM card is lost, the old SIM card can be blocked and a new card with a new IMSI, which in turn is linked to the MSISDN, can be created.