In a mobile network every subscriber is identified by a unique identifying number known as International Mobile Subscriber Identity or IMSI. IMSI is stored on the subscriber identity module (SIM) and in the HLR/AC database and in the VLR database in which area the mobile subscriber is currently roaming. IMSI's are assigned MSISDN in the HLR and subscriber related modification is done based on the IMSI of a subscriber in the HLR

IMSI Consists of three parts and is usually 15 digits long

Mobile Country Code (MCC) : 3 decimals uniquely identifying a given country
Mobile network Code (MNC) : 2 or 3 decimal places and identifies a mobile network in a country
Mobile Station Identification Number (MSIN) : 10 decimal places, used to identify a subscriber in a mobile network

In order to save a mobile subscribers from eves droppers it is avoided to transmit the IMSI over the air interface, however while doing location update it is necessary that the IMSI is transferred from the MS to the VLR and than to the HLR