What is an IMSI?
The International Mobile Subscriber Identity (IMSI) is a unique number stored on a SIM card or eSIM profile that identifies a subscriber on a mobile network. When a phone attaches to a tower, the network uses the IMSI, together with a secret key stored on the SIM, to look up the account and authenticate it. Move the SIM to another phone and the IMSI, and the subscription, go with it.
How an IMSI is built
An IMSI is up to 15 digits long and has three parts:
| Part | Digits | Meaning |
|---|---|---|
| MCC (mobile country code) | 3 | The country; 310 is one of the codes for the United States |
| MNC (mobile network code) | 2 or 3 | The carrier within that country |
| MSIN (mobile subscription identification number) | The rest | The individual subscriber |
The IMSI is not the phone number. The number people dial (the MSISDN) is linked to the IMSI in the carrier's records, which is how a number can move to a new SIM.
IMSI vs. IMEI vs. ICCID
| Identifier | Identifies | Where it lives |
|---|---|---|
| IMSI | The subscriber (the account on a carrier) | On the SIM or eSIM profile |
| IMEI | The phone hardware | Built into the device; dial *#06# to display it |
| ICCID | The SIM card itself (its serial number) | Printed on and stored in the SIM; usually 19 or 20 digits starting with 89 |
Phones show the IMEI and ICCID in their About or SIM status screens; the IMSI is often not displayed because it works behind the scenes. Devices with eSIM also have an EID that identifies the embedded SIM chip.
Privacy and security
Because the IMSI permanently identifies a subscriber, networks avoid sending it over the air more than necessary. They assign temporary identifiers instead (such as the TMSI), and 5G encrypts the permanent identity before it's sent. Fake cell towers called IMSI catchers try to trick phones into revealing their IMSI so a subscriber can be tracked, often by pushing phones down to older network generations.
For the CompTIA A+ exam
Core 1 (220-1201) objective 1.3 covers configuring cellular connectivity, including SIM and eSIM. IMSI isn't named in the objectives or the acronym list, but keeping the three identifiers straight helps with SIM questions: IMSI is the subscriber, IMEI is the device, and ICCID is the SIM card.