A telephone numbering plan is a type of numbering scheme used in telecommunication to assign telephone numbers to 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 the public 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 assigned to every telephone, while an open numbering plan features a variable length of telephone numbers assigned to stations. An open plan permits the expansion of the total numbering capacity of the plan by addition of more digits to a subset of numbers. Many numbering plans subdivide their territory of service into geographic regions designated by a special prefix, often called area 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.
The International Telecommunication Union (ITU) has established a comprehensive numbering plan, designated E.164, for uniform interoperability of the networks of its member state or regional administrations. It is an open numbering plan, however, imposing a maximum length of 15 digits to telephone numbers. The standard defines a country calling code (country code) for each state or region which is prefixed to each national numbering plan telephone number for international destination routing.
Private numbering plans exist in telephone networks that are privately operated in an enterprise or organizational campus. Such systems may be supported by a private branch exchange (PBX) which controls internal communications between telephone extensions.
In contrast to numbering plans, which determine telephone numbers assigned to subscriber stations, a dial plan establishes the customer dialing procedures, i.e. the sequence of digits users are required to dial to reach a destination. Even in closed numbering plans, it is not always necessary to dial all digits of a number. For example, an area code may often be omitted when the destination is in the same area as the calling station.
The North American Numbering Plan is a closed numbering plan which prescribes ten digits for each complete destination routing code that is divided into three parts. The most significant part is a three-digit Numbering Plan Area (NPA) code (area code). Within each plan area central offices are numbered with a three-digit central office (CO) code, the second part. The remaining four digits number the specific line assigned to each telephone. Other countries with open numbering plans may use variable-length numbers; in some, such as Finland, subscriber numbers may vary in length even within a local exchange.
From : Wiki