Area Covered by the NANP
The North American Numbering Plan (NANP) spells out how telephone numbers can be
allocated in North America and some of the "adjacent" areas. Whether you live in
New Mexico, Nunavut, the Cayman Islands, or Grenada, the NANP defines what your
area code is and -- within that -- what exchanges prefixes can be used.
The format for telephone numbers in North America is relatively straight forward.
The first three digits identify the are code, the next three digits identify the
local exchange (also known as a "central office" or "CO") and the last four digits
identify individual phone lines.
Most area codes belong to specific geographical areas. Some area codes cover large
geographic areas (867 cover millions of square miles, most of which are frozen) while
others cover relatively tiny areas (671 covers the micronesian island of Guam which
is all of 212 square miles). Some area codes, such as 710 (US Government), 800 (toll-free)
or 900 (premium sevices), are not associated with specific geographic regions.
As the population grows and develops a need fax machines, cell phones, pagers, and
extra lines for the teenager in the family, the pool of unused telephone numbers --
theoretically just under 10 million but in practice much less due to certain numbering
restrictions -- is depleted, and a new area code must be set up. At that point a
decision needs to be made: overlay or split. Both have the pro's and con's and both
cause some degree of pain to callers.
Area Code Overlays
In the Overlay scenario, the new are code covers the same geographical are as
the old area code. The new area code represents a pool of new telephone numbers which
can be handed out to new customers.
Old customers keep their telephone numbers, which
is nice in the first few years, especially if you get a lot of calls from other area
However this technique creates an additional burden for the rest of time because
you can no longer deduce somebody's area code from their phsical localtion. Your new
next door neighbour may be in a different area code, or they may not. This means that
there is no longer such a thing as a seven digit local number.
Area Code Splits
In the Split scenario, the geographical area of the old area code is divided
into two portions and all the telephones in the one of the two portions are "moved
over" to the new area code. Those exchange prefixes whose telephones were moved can
now be assigned new telephones in the old area code, and the prefixes whose telephones
did not move can be assigned new phones in the new area code.
In the short term, there
is some annoyance as callers from other area codes update the area code portion of the
callees phone numbers (for those regions that were mapped to the new area code), but
in the long term you maintain the benefit of being able to use seven digit numbers
You can split an area code several times and maintaining the ability to have seven
digit local numbers, but once you do an overlay, that ability is lost forever.
Special Area Codes
The NANP defines several special area codes that are not associated with
geographical areas. These are 456 for Inbound International calls, 500 for
Personal Communication Service, 600 for Canadian Services, 700 for Interexchange
Carrier Services, 710 for the US Government, 800/866/877/888 for Toll-Free numbers,
and 900 for Premium Services.
Special Exchange Prefixes
The NANP also reservers certain local exchange prefixes for special uses.
The "555" prefix, for instance, is reserved for information services available
in several area codes (such as "555-1212" for directory services). In Canada,
prefixes such as "604", "416", and "902", are reserved because they also happen
to be valid Canadian area codes. "911" (and 912, 913, 914, 915, and 916) also
cannot be used as an exchange prefix because they are too similar to 911