21151

21151 refers to a specific identifier used in various contexts, most notably within the **International Mobile Subscriber Identity (IMSI)** system for mobile telecommunications. An IMSI is a unique number assigned to each subscriber in a GSM (Global System for Mobile Communications) or UMTS (Universal Mobile Telecommunications System) network, typically structured as a 15-digit number. The prefix "21151" is one of the possible Mobile Country Codes (MCC) assigned to certain countries or regions.

The MCC "21151" is associated with **Taiwan (Republic of China)**, as part of its national mobile network numbering plan. The full IMSI includes the MCC followed by a Mobile Network Code (MNC) and a Mobile Subscriber Identification Number (MSIN). For example, an IMSI might appear as "211510100000001234567" where "21151" is the MCC, "01" is the MNC, and the remaining digits identify the subscriber.

In telecommunications, IMSIs are used for authentication and identification of subscribers within cellular networks. They are stored in the Home Location Register (HLR) or equivalent databases to manage subscriber data. While IMSIs are essential for network operations, they are not typically shared publicly due to privacy concerns. However, they may appear in technical documentation, network diagnostics, or regulatory filings related to mobile services.

The numbering plan for IMSIs is overseen by international organizations such as the **International Telecommunication Union (ITU)** and regional bodies like the **Asia-Pacific Telecommunity (APT)**. The allocation of MCCs ensures unique identification across global mobile networks while maintaining regional consistency.