NPA-NXX is the first six digits of a 10-digit number (NPA-NXX-XXXX)

NPA (Numbering Plan Area) is the Area Code

NXX (no acronym) is the COC (Central Office Code) or the local exchange

 

IMPORTANT - NPA and NXX use the exact same format !!!  Both NPA and NXX use the "NXX" format where  N = 2-9  (excluding 0 and 1)  and  X = any digit 0-9

All Telecom workers hear these acronyms over and over again . . . NPA-NXX, also commonly written as NPA/NXX (no idea why a slash is used).

NPA (Numbering Plan Area)

3 digit area code, as in NPA-NXX-5555 - for a given NPA, no 2 phones can have the same 7-digit number that follows the NPA.  NPA actually has the format of NXX, where N is any digit from 2 to 9 and X is any digit from 0 to 9.

NXX (Central Office Code  or  Local Exchange)

The N and X just refer to digits, not an acronym).  NXX is a 3 digit local exchange, as in NPA-NXX-5555, where N=any digit except 0 and 1 (2 to 9), and X=any digit (0 to 9). N cannot be “1”, because the LEC will mistake a local call for a long distance call. The plan is to eventually go to 10 digit dialing for all national and local calls, which will allow N to be a “1”, and then the local exchange will be denoted as “XXX”.NXX (the N and X just refer to digits, not words as NNX does)

NOTE: NXX replaced NNX (Network Numbering eXchange) - the old local exchange, which limited the 2nd digit to only allow digits from 2 to 9. This has been replaced by NXX, which allows the 2nd digit to be anything. NNX has only 640 possible combinations, whereas NXX has 792.

XXXX (the extension)

X = 0-9

The XXXX extension follows the NPA-NXX digits, and can be any number between 0000 and 9999.  For example, a switch serving +1-202-533 would typically serve +1-202-533-0000 through +1-202-533-9999. In major cities, switches usually host several CO codes.