The North American Numbering Plan (NANP) administers telephone numbers across multiple countries, including the United States. A telephone number in the U.S. consists of a three-digit area code, a three-digit central office code (also known as an exchange code), and a four-digit subscriber number. Thus, each individual phone number is ten digits long. This structure dictates the potential number of unique telephone numbers that can be assigned.
The structure allows for a large, yet ultimately finite, quantity of phone numbers. The capacity is determined by the number of possible combinations for each code. The area codes and exchange codes are subject to specific rules and restrictions which impact their availability and influence the actual count of usable numbers. Historical context shows how the system evolved to accommodate increasing demand for phone services, which led to the introduction of new area codes and number pooling mechanisms to maximize efficient resource utilization.
