An International Mobile Subscriber Identity or IMSI is an individual number related with all Global System for Mobile Communications (GSM) and Universal Mobile Telecommunications System (UMTS) network mobile phone users used for recognizing a GSM subscriber.
The IMSI is containde in the Subscriber Identity Module (SIM) inside the phone and is sent by the phone to the needed mobile network.
All IMSI numbers consist of two parts- the initial part contains six digits in the North American standard and five digits in the European standard. It recognizes the user’s GSM network operator in a particular country. The second part is allocated by the network operator to uniquely distinguish the subscriber.
The IMSI is a significant part of the Home Location Register (HLR) lookup operation as it is needed to recognize to what network a mobile number belongs to and that’s why the process is sometimes called IMSI Lookup in addition to HLR Lookup or Number Lookup.