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According to 47 CFR 52.20, a US federal regulation administered by the Federal Communications Commission: [30] Thousands-block number pooling is a process by which the 10,000 numbers in a central office code (NXX) are separated into ten sequential blocks of 1,000 numbers each (thousands-blocks), and allocated separately within a rate center.
NANP telephone numbers are usually rendered as NPA-NXX-XXXX or (NPA) NXX-XXXX. For example, 250 555 0199, a fictional number, could be rendered as 250-555-0199, (250) 555-0199, 250-5550199, or 250/555-0199. The parentheses were originally used to indicate that the area code was not necessary for local dialing.
Toll-free telephone numbers in the NANP are regulated by the Code of Federal Regulations (CFR) Part 52 Section 101. RespOrgs assign the numbers in the SMS/800 database. SMS/800, Inc. administers this database as the Number Administration and Service Center, as a subcontractor for the Federal Communications Commission (FCC).
Each rate center is associated with: a geographical place name (city, province/state) a nominal physical location (V and H co-ordinates) for distance calculations for billing purposes; one or more prefixes, in the form +1-NPA-NXX, which each identify a block of ten thousand directory numbers
The Local Exchange Routing Guide (LERG) is a database of telecommunications numbering resources for use in the administration and operation of the North American Numbering Plan (NANP) by the NANP administrator (NANPA) and telecommunications service providers.
Thus, NXX is a number from 200 through 999, while XXXX is a range from 0000 through 9999. The first three digits of a telephone number are the numbering plan area code (NPA code, or simply NPA). The next three, NXX, identify the central office and the last four digits are the line number of an individual office.
The North American Numbering Plan (NANP) divides the territories of its members into geographic numbering plan areas (NPAs). Each NPA is identified by one or more numbering plan area codes (NPA codes, or area codes), consisting of three digits that are prefixed to each local telephone number having seven digits.
From 1947 to 1995, all NPA codes were distinguished with the digits 0 or 1 in the middle position. This provided the traditional format N 0/1 X, where N is any digit from 2 to 9, and X is any of the ten numerals. This format provided a set of 160 combinations, but only 144 were in use for geographic NPA codes.