What is an IMSI Number? Format and IMSI Vs IMEI?

The International Mobile Subscriber Identity(IMSI) is an identity of a subscriber in the GSM Telecom Network. The full form says that this identity is unique across the globe. To maintain uniqueness, it follows a hierarchical structure. In this tutorial, we explain the IMSI format and other details.

The length of the IMSI number is 15 digits. The IMSI number consists of MCC, MNC, and MSIN. The globe has multiple countries, each with a mobile country code or MCC.

The MCC is not the country code(CC) we use while dialing an international mobile number. Here is the list of MCC and CC  for all countries.

Each country gets the MCC from a standard telecommunication body. It is of a fixed length of three digits,

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 can be multiple mobile operators at various locations. Each operator has multiple mobile network codes or MNCs within the country. Within a nation, according to the area. The MNC specifies a PLMN for a mobile operator.

The MNC can be 2 or 3 digits long.

Suppose there is more than one mobile operator at the exact location. All will have different MNCs. After MCC, the next 2 or 3 digits are for MNC.

The remaining 10 or 9 digits are mobile subscription identification numbers (MSIN). An MSIN identifies a subscriber within a PLMN. A mobile operator is free to choose a unique MSIN.

IMSI security over the network: 

As a sim card’s primary identity,  The telecom network uses IMSI 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 as the temporary identity of the sim card over the radio interface. During the first attachment to an MME or SGSN, the core network allocates the TMSI or GUTI for further network requests.

Multi IMSI SIM card:

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 sim cards have multiple IMSIs configured. Depending on the location, the identity changes. This reduces roaming changes. A SIM with numerous identities is also called a roaming SIM card. Many MVNOs use multi-IMSI sim cards for low-cost roaming.

IMSI vs. Imei

Similar to IMSI, IMEI is another unique international number. But they are different identities. IMEI identifies a hardware device, while IMSI is for a mobile subscriber. IMEI vs. IMEI is a comparison in simple terms.

  • 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.
  • Device manufacturers use IMEI, while the mobile network operator uses IMSI.
  • SIM cards come with a validity time. After that, a mobile operator can reuse the IMSI. While IMEI is not reusable.