An IMSI is used in any mobile network that interconnects with other networks. This unique, 15-digit international identifier, which is included in a mobile device, allows for roaming on mobile service provider networks. An IMSI includes three components: Three-digit Mobile Country Code (MCC), three-digit Mobile Network Code (MNC) and nine-digit Mobile Station Identification Number (MSIN).
The IMSI Oversight Council (IOC) is a committee of the Alliance for Telecommunications Industry Solutions (ATIS) that oversees the performance of the IMSI administration. Revisions to the guidelines are effective upon approval by the IOC. ATIS is the United States IMSI Administrator, which administers the HNI and IBN on behalf of the Department of State. The MSIN is administered by the network operator to which the HNI is assigned.
More information can be found in the IMSI Assignment Guidelines and Procedures, which are maintained by the IOC.