IMSI, or International Mobile Subscriber Identity, is a unique identifier used by mobile networks to recognise subscribers and manage connectivity. It plays a key role in how SIM cards authenticate devices, select networks, and maintain reliable connections. This guide explains what IMSI is, how it works, and why it matters for modern mobile and IoT connectivity.
Most people never think about the numbers stored inside a SIM card, yet these numbers decide how a device connects to a network, how it authenticates, and even which carrier it uses. One of the most important of these identifiers is the IMSI. If you use IoT devices in your operations, or if you manage systems that rely on mobile connectivity, understanding IMSI will help you see how network access really works behind the scenes.
IMSI plays a central role in everything from standard cellular communication to advanced technologies such as Multi-IMSI SIMs. In this guide, we take a closer look at what the IMSI actually is, how this number is put together, and why it plays such an important role in keeping today’s connected devices online.
Table of Content
- What Does IMSI Mean
- How the IMSI Number is Structured
- How IMSI Works When a Device Connects
- IMSI vs ICCID vs IMEI
- Why IMSI Matters in IoT Connectivity
- How Multi-IMSI SIMs Use Multiple IMSI Profiles
- How eSIM and eUICC Handle IMSI Profiles
- Security Considerations for IMSI
- FAQ About IMSI
What Does IMSI Mean
IMSI is short for International Mobile Subscriber Identity. It is a number that identifies the mobile subscriber and not the device itself. When a SIM tries to connect to a network, this is the number the carrier looks at to recognise the subscriber and the operator responsible for that connection.
The IMSI sits inside the SIM or the eSIM profile. When a device starts up and reaches out to a network, the carrier checks this number as part of the normal authentication step to confirm that the subscriber is allowed to use the service.
You may have seen other SIM identifiers, such as the ICCID. Each one has its own role. The ICCID belongs to the physical SIM card. The IMSI is used inside the network to identify the subscriber. Both identifiers are needed, and together they help the activation process work as expected.
How the IMSI Number is Structured
At first glance, an IMSI looks like a simple string of numbers, but each part of it has a specific role. The number is built from three sections that together tell the network where the subscriber belongs and how they should be recognised.
The IMSI starts with the Mobile Country Code, usually shortened to MCC. This section points to the country linked to the operator itself. It does not change when the device moves or travels. Instead, it stays tied to the subscriber’s home network, regardless of where the connection happens.
Next comes the Mobile Network Code, or MNC. This section identifies the carrier within that country. In markets with several operators, the MNC helps the network distinguish between them and route the connection correctly.
The final part is the Mobile Subscriber Identification Number, known as MSIN. This is the portion that makes the IMSI unique. It is assigned by the operator and differentiates one subscriber from another, even when they use the same carrier.
In most cases, an IMSI contains fifteen digits in total. The MCC and MNC follow global standards so that networks around the world can interpret them correctly. The MSIN is created by the operator and is unique for each subscriber. This is why two SIM cards from the same provider will never share the same IMSI.
This structure allows mobile networks to identify subscribers quickly and consistently. With this number alone, the network can determine the home operator and begin the authentication process needed to bring a device online.
How IMSI Works When a Device Connects
When an IoT device or mobile terminal powers on, it must identify itself before it can use a mobile network. The IMSI stored on the SIM is used at this stage to tell the network who the subscriber is and which operator should manage the connection.
When the device reaches a nearby cell tower, the IMSI is checked as part of the initial network handshake. The carrier uses it to confirm that the subscriber exists in its systems and is authorised to connect. If the information does not line up, the connection simply does not move forward.
In addition to the IMSI, the SIM also contains a private security key known as Ki. The key stays on the SIM itself and does not leave it. When the device connects, the network checks the response it gets back, rather than exchanging the key or sending sensitive data back and forth.
Once the connection is up and running, the IMSI steps out of the picture. The network switches to short-lived identifiers instead, so the subscriber identity is not constantly reused while the session is active.
For IoT deployments, this process happens whenever a device reconnects, moves between coverage areas, or recovers after a network interruption. Because many IoT devices operate without human oversight, a consistent and reliable IMSI-based authentication process is essential for keeping connections stable.
IMSI vs ICCID vs IMEI
These are three identifiers that often get mixed up because they all appear in mobile and IoT connectivity. While they are related, each one serves a very different purpose within the network.
The IMSI is tied to the subscriber. It is the identifier the mobile network uses to recognise who is requesting access and which operator should handle the connection. IMSI plays a central role during authentication and determines how the device is treated by the network.
The ICCID belongs to the SIM card itself. It identifies the physical or embedded SIM and is used for activation, provisioning, and inventory tracking. This is the number operators refer to when a SIM is issued, replaced, or managed internally, rather than when dealing with the device or the subscriber.
The IMEI identifies the device hardware itself. It does not change when a SIM is swapped and is handled separately from both the subscriber and the SIM. As a result, networks treat the device as its own element, independent of who is using it or which SIM is installed.
When working with IoT devices, these distinctions matter. Some checks happen at the subscriber level, others at the SIM level, and others at the device level. Knowing which identifier applies in each case makes it easier to understand what is happening when a connection fails or behaves unexpectedly.
Why IMSI Matters in IoT Connectivity
IoT devices rely on cellular connectivity to do their job. Whether it is processing payments in retail, monitoring equipment, providing internet backup, or keeping digital signage online, the connection needs to be available when the device is in use. If a device cannot authenticate properly, or if its IMSI is tied to a carrier with limited coverage, that connection can fail without warning.
Network quality is not the same everywhere. In some areas, coverage can be inconsistent or depend heavily on a single operator. When a device relies on just one IMSI linked to one carrier, it becomes vulnerable to local outages, congestion, or gaps in coverage. In those situations, a loss of connectivity can take the device offline until the network recovers.
This challenge becomes more visible when IoT devices are deployed at scale or across borders. Managing connectivity in different countries often means dealing with different carriers, regulations, and coverage conditions. IMSI-based solutions that work with more than one network are often used to handle these differences more easily. A single SIM setup can be used across regions, reducing the need to manage different connectivity options for each market.
From an operational point of view, this also affects logistics. When fewer SIM variants are involved, inventory becomes easier to manage and device setup takes less time. Devices can usually be shipped and activated without adjusting the connectivity configuration for each location.
This is one of the reasons Multi-IMSI SIMs are commonly used in large and mission-critical IoT deployments. By allowing devices to authenticate through more than one network, they help maintain connectivity in environments where reliability is essential.
How Multi-IMSI SIMs Use Multiple IMSI Profiles
A Multi-IMSI SIM stores more than one IMSI on the same card. Each IMSI is linked to a different mobile network. When a device connects, it does so using one of those identities. If the connection does not succeed, the device can try again using another IMSI that points to a different carrier.
This comes into play in areas where coverage is inconsistent or where network performance changes from place to place. In those situations, the device is not limited to a single operator and can try another available network.
In many IoT deployments, IMSI profiles are not fixed forever. With eUICC support, profiles can be changed remotely without touching the device or replacing the SIM. This is often used when devices move between regions, when network agreements change, or when performance on a particular carrier starts to degrade.
Rather than relying on one predefined setup, the SIM can adapt over time. The goal is not to optimise every connection, but to avoid situations where a device stays offline simply because one network is no longer a good fit.
How eSIM and eUICC Handle IMSI Profiles
eSIM makes it possible to change the IMSI without swapping out the SIM. It is delivered to the device digitally and can be changed later without swapping hardware. This is useful when devices move between regions or when connectivity requirements shift over time.
eUICC is the component inside the device that handles those profiles. It keeps track of which IMSI is active and allows profiles to be updated or replaced remotely, even when managing large numbers of devices.
For large IoT projects, using eSIM together with eUICC removes many of the limits associated with traditional SIM cards. Devices can be deployed with the same setup and adjusted later as needed, which simplifies rollouts and reduces the effort involved in managing connectivity across countries.
Security Considerations for IMSI
IMSI is used to identify the subscriber during the initial connection to a mobile network. For that reason, it is not meant to be exposed repeatedly during normal communication. Once authentication is completed, networks rely on temporary identifiers instead of using the IMSI directly.
Additional protections are commonly used in IoT environments. Private APNs, VPN connections, and encrypted tunnels help limit where subscriber information is visible and how data moves across the network. These measures are part of standard mobile network operation and are especially relevant for devices that communicate continuously or operate without supervision.
For businesses using cellular connectivity, how IMSI is handled after the initial connection can affect both security and reliability.
Frequently Asked Questions About IMSI
Below are answers to common questions about IMSI, including privacy, security, and how it is used in mobile and IoT connectivity.
IMSI is used by mobile networks to recognise a subscriber during the initial connection. It is not meant to be used as a tracking tool. After a device connects, the network switches to temporary identifiers, so the IMSI is not constantly reused.
In IoT setups, connections are often handled through private APNs or secured network paths. Because of that, IMSI exposure is limited and tracking is not a typical concern for managed IoT deployments
An IMSI catcher is equipment that behaves like a mobile base station and tries to collect subscriber identifiers from nearby phones. Most discussion around IMSI catchers focuses on mobile phones and public networks.
IoT devices usually operate in more controlled environments. Private network configurations and encrypted connections reduce how often subscriber identifiers are visible, which makes this topic less relevant for most IoT use cases.
No. The IMSI refers to the subscriber within the mobile network. The SIM card number, known as the ICCID, refers to the SIM itself.
Both numbers exist on the SIM, but they are used for different reasons. The ICCID is used when SIMs are issued, activated, or managed. The IMSI is used by the network when a device connects. They are related, but they are not interchangeable.
Yes. An eSIM uses IMSI profiles in the same way a physical SIM does. The difference is how those profiles are handled.
With eSIM, IMSIs can be added or changed without replacing hardware. This is useful when devices are deployed across regions or when connectivity needs change over time.
Some devices operate in locations where coverage varies or where a single network is not always reliable. In those cases, relying on one IMSI can lead to downtime.
Multi IMSI SIMs give devices access to more than one network identity. If one carrier cannot provide a stable connection, another can be used instead. This approach is common in IoT systems that are expected to stay online, such as payment terminals, digital displays, charging infrastructure, and monitoring equipment.
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