Download - Telephone Numbering Plan
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.