International mobile subscriber identity

The international mobile subscriber identity is a number, that uniquely identifies a device on a mobile network. The name is often shortened to IMSI. The dsevice will send this number to identify itself on a mobile network. One occasions when it does this is when it is powered on. The number is stored on a special microchip, which is called SIM card. These numbers are unique, and they are not linked to the phone number the device uses. The microchip will store the number securely. It also has cryptographic functioons: It can generate public and private keys to use for communication. In total, an IMSI has a length of 64 bit. Usually, this ocrresponds to a 14 or 15 digit number. They have apredefined form, certain positions n the number have a special meaning. That way, the three digits at the start of the number identify the country. The next two or three digits identify the network provider, the telecommunications company. The rest of the number is used to identify the subscriber. 

There is another number, the  Temporary Mobile Subscriber Identity (TMSI), which is commonly used in communication. Much like the IMSI, it allows to identify a subscribner. It is not a number that is unique world-wide. It only has a limited time of validity. In the easiest case, it is randomly generated. The most common use of the TMSI is for paging a mobile device.