Insights · Updated 2026-08
IMSI explained: how MCC and MNC identify your SIM
The IMSI — International Mobile Subscriber Identity — is the number that says who a SIM belongs to. It is provisioned by your home operator, stored on the SIM or eSIM, and never printed on the outside of the card. Its opening digits are the same MCC and MNC you find everywhere else in mobile numbering, which is exactly how a network anywhere in the world can tell which operator to route your traffic back to.
Anatomy of an IMSI
An IMSI is at most 15 digits and breaks into three parts defined by ITU-T E.212:
| Part | Digits | Meaning |
|---|---|---|
| MCC | 3 | Home country of the subscription |
| MNC | 2–3 | Home operator within that country |
| MSIN | 9–10 | The individual subscriber, assigned by the operator |
The MCC and MNC together are the PLMN; the MSIN (Mobile Subscriber Identification Number) is the operator's private serial for that line. Because the total is capped at 15, a three-digit MNC leaves room for a nine-digit MSIN and a two-digit MNC leaves ten.
├─ 230 ──────── MCC → Czech Republic
├─ 02 ───────── MNC → O2
└─ 1234567890 ─ MSIN → this subscriber
Read the front of any IMSI and you have the operator: 23002… is O2 in the Czech Republic, while 310410… is AT&T in the United States. You can confirm the PLMN prefix in the registry or via the API.
IMSI is not your phone number
Three identities are easy to confuse. The IMSI identifies the subscription; it is not what people dial, and it does not identify the handset.
| Identifier | Identifies | Lives on |
|---|---|---|
| IMSI | The subscription / SIM | The SIM, in signalling |
| MSISDN | The dialable phone number | The operator's records |
| IMEI | The physical handset | The device hardware |
A SIM can keep the same IMSI while its MSISDN changes (number porting), and it can move between handsets, each with its own IMEI. Only the IMSI carries the MCC/MNC that names your home network.
How the MCC and MNC route you home
When you power on abroad, the visited network reads the MCC and MNC at the start of your IMSI, looks up the corresponding home network, and signals it to authenticate you and authorise service. That single lookup is why roaming works at all: your identity is portable because its first five or six digits unambiguously name one operator on the planet — the mechanism described in how to read a PLMN.
Why the IMSI is kept hidden
Because the IMSI is a stable, unique identity, exposing it is a privacy risk — historically an "IMSI catcher" could impersonate a cell to make nearby phones reveal theirs. To limit this, networks hand out temporary identities (TMSI, and GUTI in LTE) for most exchanges, so the real IMSI is sent rarely. In 5G the permanent identity is renamed the SUPI and is transmitted only in an encrypted form, the SUCI, so it is never broadcast in the clear. The MCC and MNC structure, though, is unchanged — the home network still has to know which country and operator to consult.