Atelephone numbering planis a type ofnumbering schemeused intelecommunicationto assigntelephone numbersto subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, reachable by a system of destination code routing. Telephone numbering plans are defined in each of administrative regions of thepublic switched telephone network(PSTN) and they are also present in private telephone networks.Numbering plans may follow a variety of design strategies which have often arisen from the historical evolution of individual telephone networks and local requirements. A broad division is commonly recognized, distinguishing open numbering plans and closed numbering plans. A closed numbering plan imposes a fixed number of digits to every telephone number, while an open numbering plan allows variance in the numbers of digits. Many numbering plans subdivide their territory of service into geographic regions designated by anarea code, which is a fixed-length or variable-length set of digits forming the most-significant part of the dialing sequence to reach a telephone subscriber.TheNorth American Numbering Plan(country code1) is a closed numbering plan[1][unreliable source?]which prescribes ten digits for each complete destination routing code. A complete telephone number consists of three parts. The most significant part is a three-digit Numbering Plan Area (NPA) code (area code). Each area code comprises a set of three-digit central office (CO) codes, which are unique to eachtelephone exchangewithin an NPA. The remaining four digits are the specific station number assigned to each telephone. Other countries with open numbering plans may use variable-length numbers; in some, such asFinland, subscriber numbers may vary in length even within a local exchange.