IMSI or International Mobile Subscriber Identity
IMSI full form says that it is the identity of a mobile subscriber that is unique across the globe. The length of the IMSI number is 15 digits. For uniqueness, it follows a hierarchical structure. The IMSI number structure consists of MCC, MNC, and MSIN. The glob has multiple countries, each has a mobile country code or MCC. Mobile country code is different from country code or CC, the MCC is assigned to each country from telecommunication standard body. It is of fix length which is three. E.g 404 is the MCC of India, while 91 is the county code. The first three digits of IMSI are MCC.
Within a country, there are multiple mobile operators at multiple locations. Each operator has multiple mobile network codes or MNCs, within a country according to location. The MNC specifies a PLMN for a mobile operator. The MNC can be 2 or 3 digits long. If there are more than one mobile operator is present at the same location will be having different MNCs. The next 2 to 3 digits are MNC code. The remaining digits 10 or 9, are mobile subscription identification number (MSIN), A MSIN identifies a subscriber within a PLMN.
Security on the network:
It is the primary identity of a sim card. The mobile network uses this identity for calls and SMS. If someone knows the IMSI and has software to hack the call or SMS, he might succeed on the radio interface. For security reasons, the radio network uses TMSI. Which is the temporary identity of the sim card over the radio interface?
Multi IMSI :
When a person buys a sim card. It comes with an IMSI or unique identity, which belongs to the network operator of the sim card. Most of the time this does not change. But there are sim cards, those may have multiple IMSIs configured in a SIM. Depends on the location, the identity changes. This reduces roaming changes.
Imsi vs Imei
Most of the time its IMSI is confusing with the IMEI. But they are totally different parameters. When a mobile subscriber changes the sim card, IMSI changes but the IMEI remains the same. IMEI is the 14 Digits long field, while the other is 15 digits long. IMEI is used by device manufacturers while other is used by the mobile network operator.