The international mobile subscriber identity (IMSI; /ˈɪmziː/) is a number that uniquely identifies every user of a cellular network.
To prevent eavesdroppers from identifying and tracking the subscriber on the radio interface, the IMSI is sent as rarely as possible and a randomly-generated TMSI is sent instead.
Outside North America, the IMSI is converted to the Mobile Global Title (MGT) format, standard E.214, which is similar to an E.164 number.
E.214 provides a method to convert the IMSI into a number that can be used for routing to international SS7 switches.
E.214 can be interpreted as implying that there are two separate stages of conversion; first determine the MCC and convert to E.164 country calling code then determine MNC and convert to national network code for the carrier's network.
The result is an E.214 compliant Global Title, (Numbering Plan Indicator is set to 7 in the SCCP message).
The Home Network Identity (HNI) is the combination of the MCC and the MNC.