# What is an IMSI?

> IMSI (International Mobile Subscriber Identity) is the globally unique identifier a mobile network uses to recognise a subscriber. Stored on the SIM/eUICC and in the HLR/HSS, it is up to 15 digits: a Mobile Country Code (MCC), a Mobile Network Code (MNC), and a subscriber identity (MSIN). Unlike the MSISDN — the dialable phone number — the IMSI lives on the signalling plane and is never exposed to other subscribers.

- Canonical URL: https://www.broadnet.me/answers/what-is-imsi
- Language: en
- Last reviewed: 2026-07-22
- Publisher: BroadNet (https://www.broadnet.me)

## IMSI vs MSISDN

The MSISDN is the external routing number; the IMSI is the internal network identity. The two are decoupled: MNP lets a subscriber keep their MSISDN while their IMSI stays with the SIM. An SS7 SendRoutingInfoForSM query returns the recipient's IMSI plus the serving MSC — which is exactly why that operation is a fraud target. See /answers/what-is-msisdn and /answers/ss7-map-send-routing-info-for-sm.

## Why the IMSI is sensitive

Because the IMSI ties directly to a real subscriber, its exposure enables tracking (the classic 'IMSI-catcher' attack) and SMS interception. 5G conceals the permanent identity behind the SUPI/SUCI scheme so it is never sent in the clear. Operator signalling firewalls guard IMSI-revealing queries at the network edge. See /operators/ss7-firewall.

## Related

- [All answers](https://www.broadnet.me/answers)
- [Glossary](https://www.broadnet.me/answers)
- [What are MCC and MNC codes?](https://www.broadnet.me/answers/what-is-mcc-mnc)
- [What is an MSISDN?](https://www.broadnet.me/answers/what-is-msisdn)
- [What is an IMEI?](https://www.broadnet.me/answers/what-is-imei)
- [MAP-SRI-for-SM — Send-Routing-Info-for-SM, and SS7 SMS security](https://www.broadnet.me/answers/ss7-map-send-routing-info-for-sm)
- [eUICC — the embedded UICC chip explained](https://www.broadnet.me/answers/what-is-euicc)
- [SS7 Firewall](https://www.broadnet.me/operators/ss7-firewall)
- [eSIM VSM platform](https://www.broadnet.me/operators/esim-vsm)