IMSI: The central subscriber identification explained

The IMSI number uniquely identifies every mobile subscriber within a network. Companies offering mobile services and IoT applications need the IMSI to ensure secure and reliable communication between devices and networks. In this article, you’ll learn everything essential – from its function and security aspects to the relevance of IMSI, eSIM, and 5G in IoT environments.

What is IMSI?

IMSI stands for ‘International Mobile Subscriber Identity’ and works similarly to a fingerprint for mobile subscribers. It is a globally unique identification number assigned to a mobile subscription and stored in a SIM, eSIM, or iSIM profile. Mobile users or mobile subscriptions are identified with it within a mobile network. 

 The IMSI SIM number contains information about the country, the network operator and the individual subscriber identification, allowing the user to be clearly assigned to a mobile provider, which allocates network settings such as tariff plans and permissions.

For IoT and M2M, the IMSI number plays a crucial role. Thanks to its seamless integration into global mobile standards, the number offers flexibility and security, meaning IoT devices can operate worldwide in mobile networks, given that network coverage, roaming agreements, supported radio technologies, and regulatory requirements are in place. 

The role of the International Mobile Subscriber Identity in mobile networks

The IMSI uniquely identifies mobile subscribers. As soon as a subscriber connects to a mobile network, the device sends the IMSI SIM number to the network operator, who confirms the identity. This step then grants access to network resources and assigns the correct tariff.

Another critical application is authentication. The IMSI number ensures that only authorized users can access the mobile network, protecting both the subscriber and the operator from unauthorized access. After authentication, the IMSI correctly assigns mobile usage and ensures accurate billing.

The role of the IMSI SIM Number on the Internet of Things

The IMSI uniquely identifies mobile subscribers or the corresponding mobile subscription. As soon as a subscriber connects to a mobile network, the device sends the IMSI or a temporary or protected subscriber identifier  to the network operator. The operator can then verify the identity of the subscription and prepare access to the network. This step then grants access to network resources and assigns the correct tariff.

Another critical application is authentication. The IMSI itself does not directly authenticate the user. Instead, it helps the network find the correct subscriber record in the home network. The actual authentication then takes place through cryptographic procedures and secret keys stored in the SIM/USIM profile and in the mobile operator’s home network. This protects both the subscriber and the operator from unauthorized access. After authentication, the IMSI correctly assigns mobile usage and ensures accurate billing.

IMSI in 5G: SUPI and SUCI

In 5G, the permanent subscriber identifier is known as the SUPI (Subscription Permanent Identifier). An IMSI can be one form of this permanent subscriber identifier. To protect the subscriber’s identity, 5G uses the SUCI (Subscription Concealed Identifier). It is a protected form of the SUPI and prevents the permanent subscriber identifier from being openly transmitted over the air interface.

This mechanism improves data protection compared with older mobile generations, where the IMSI could be exposed under certain circumstances. For companies using IoT or M2M applications, this is relevant because many devices remain in the field for long periods and must be protected against misuse and unauthorized network access.

The structure of the IMSI code explained

The length of the IMSI consists of up to 15 digits and is composed of three main components: the Mobile Country Code (MCC), the Mobile Network Code (MNC), and the Mobile Subscriber Identification Number (MSIN).

• Mobile Country Code (MCC): The MCC is made up of three digits and specifies the country where the mobile subscriber is registered. For example, 232 represents Austria.
• Mobile Network Code (MNC): After the MCC comes the MNC, typically consisting of two digits. This code identifies the specific mobile operator within the country. In Austria, the code 02 represents A1 Telekom Austria.
• Mobile Subscriber Identification Number (MSIN): The last and largest section of the IMSI number is the MSIN, which has a total of nine or ten digits and uniquely identifies the subscriber within a network. This allows millions of mobile subscribers to be individually recognized within a network.

Together, they complete the full form of the IMSI number, which is globally unique for each subscriber or mobile subscription. It not only supports the unique assignment of mobile subscriptions, but also forms the basis for roaming, network registration and international connectivity. This is particularly important for international IoT and M2M applications.

The role of the IMSI SIM Number on the Internet of Things

In connected environments, it’s crucial that each device is reliably and uniquely identified and can securely communicate with networks and other systems. IMSI ensures that IoT devices receive unique identification and secure access to network resources.

For M2M (Machine-to-Machine), the IMSI supports automated communication between machines. Sensors, vehicles, or industrial systems can exchange data via mobile networks, which are processed by central platforms. The IMSI number enables global connectivity, allowing devices to communicate worldwide, regardless of their location – provided that the respective networks, roaming profiles, and radio standards such as LTE-M, NB-IoT, or 5G are supported.

The authentication process ensures that only authorized devices access the network and that data is transmitted securely. This security is especially important in critical infrastructures such as energy supply or logistics.

In IoT environments, eSIM technology also plays an important role. With an eUICC, mobile profiles can be provisioned, changed, or managed remotely. This allows device fleets to be activated at scale and adapted throughout their lifecycle to new network operators, deployment countries, or connectivity requirements.

Security concerns about the International Mobile Subscriber Identity

Exposing the IMSI code can pose a security risk, as attackers are able to track a device’s location or monitor the user’s activities. To counter this, mobile networks use temporary or protected subscriber identifiers to avoid exposing the permanent IMSI unnecessarily. Once a device connects to the mobile network, a temporary identifier often replaces the IMSI in older mobile generations. This is called TMSI (Temporary Mobile Subscriber Identity). This temporary code changes regularly, ensuring that the actual IMSI number is not revealed in every communication process.

In LTE and 5G, further temporary or protected identifiers are used, such as GUTI or 5G-GUTI. In 5G, the SUCI additionally protects the permanent subscriber identifier. Nevertheless, the IMSI remains a sensitive identifier that should not be unnecessarily exposed or stored without encryption.

Conclusion: The central role of IMSI in connectivity

The IMSI number is indispensable for identifying mobile subscribers or mobile subscriptions and forms an important basis for authentication, billing, and seamless connections in global mobile networks. Especially in the Internet of Things, companies use the IMSI to manage and protect a variety of connected devices. Together with eSIM/eUICC, Remote SIM Provisioning, Multi IMSI, and modern 5G security mechanisms, it remains a central element for scalable, international IoT connectivity. It will continue to be an essential element for mobile communication.