IMSI or International Mobile Subscriber Identity
As IMSI full form says that it is the identity of a mobile subscriber which is unique across the glob. The length of imsi number is 15 digits. For uniqueness , it follows a hierarchical structure. The imsi number structure consists of MCC, MNC and MSIN. The glob have multiple countries, each have 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. First three digits of IMSI are MCC.
With in a country there are multiple mobile operators at multiple locations. Each operator have multiple mobile network codes or MNCs , with in 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 a more than one mobile operators are present at same location will be having different MNCs. 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 with in a PLMN.
Security on network:
It is the primary identity of a sim card. Mobile network uses this identity for calls and sms. If some one know the IMSI, and have software to hack the call or sms, he might succeed on radio interface. For security reasons, radio network uses TMSI. Which is the temporary identity of the sim card.
Multi IMSI :
When a person buys a sim card. It comes with a imsi or identity, while 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 it. Depends on the location, the Identity changes. This reduces the 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 sim card, IMSI changes but the IMEI remains same. IMEI is the 14 Digits log field , while other is 15 digits long. IMEI is used by device manufactures while other is used by mobile network operator.