IMSI stands for International Mobile Subscriber Identity, and it is a unique number that identifies each user of a cellular network. It is stored as a 64-bit field and is sent by the mobile device to the network.
An IMSI is typically presented as a 15-digit number, although it can be shorter. The first three digits represent the country code of the mobile service (MCC), followed by the mobile network code (MNC), which can consist of two digits (European standard) or three digits (North American standard). The remaining digits make up the mobile subscriber identification number (MSIN) within the network's customer database, usually with a length of 9 to 10 digits, depending on the MNC's length.
The IMSI adheres to the E.212 numbering standard of the ITU, ensuring its standardization in telecommunications networks worldwide.