The North American Numbering Plan

The network numbering plan was designed to allow for the quick and discreet connection to any phone in the country. The North American numbering plan, as it is called, works on a series of 10 numbers.

The Area Code

Note that there have been some changes in this numbering plan. When it originally was formulated, the telephone numbers were divided into three sets of sequences. The first was a three-digit area code [or numbering plan assignment CNPA)]. This started with a digit from 2 to 9 in the first slot of the sequence. In the second slot, the number was set as a 0 or a 1. In the third slot, it could be any digit from 1 to 0 (0 being the 10 digit). The reasons behind this sequence were very clever. For example, the first digit did not use a 0 or 1 because these digits were used for access to operators (0) or operator services such as credit card calling, etc. The 1 was used to send a significant digit to the local switching office indicating that the call was long-distance; therefore, the switch could immediately start setting up a toll connection to the toll center or tandem office. Thus, the exclusion of 0 and 1 in the first digit of the area code facilitated the quicker call set-up. In the second slot the digit was only a 1 or 0. This was used by the switching office equipment in a screening mode. As soon as the system sampled the second digit and saw a 0 or 1, it knew that this three-digit sequence was an area code. The third slot was any digit, so it didn’t matter, it was just processed normally. Back in the early 1960s, we recognized that we were running out of the area codes, given that there were only 160 available (8x2x10= 160). In reality, only 152 were allowed for use by the various states because certain ones were allotted for special services (the area codes with an Nil were always reserved, e.g., 211, 311, 411, . . . etc.). Very close administration of the area code assignments held this until 1995, when the inevitable had to occur. From the outset, the use of the entire numbering plan was limited, but it held up for over one hundred years.

The North American Numbering Plan as It Evolved

Timing	Area code	central office code	Station subscriber number
Original	NO/1X	NNX	XXXX
Pre-1995	NO/1X	NXX	xxxx
Post-1995	NXX	NXX	XXXX

 

The Exchange Code

Following the area code is another three-digit sequence, called the exchange code. This is a central office designator that lists the possible number of central office codes that can be used in each area code. The exchange code originally was set up in the sequence NNX, meaning that the first and second numbers used the digits 2 through 9, for the same reasons stated in this area code. 1 and 0 were reserved for operator and long-distance access and the 0/1 exclusion in the exchange code prevented this three digit number from being confused with an area code. In the third number slot of the exchange code, any digit could be used. Clearly the greatest limitation in the exchange codes was that we would run out of exchange codes first. With NNX we have 640 possible exchange numbers to use (8 x 8 x 10 == 640), but these were used very quickly.

In the late 1960s, we began using an exchange code that changed the sequence to NXX or expanding the number of exchange codes in each of the area codes to 800. This added some relief to the numbering plan, but when using the NXX sequence the need arose for a forced 1 in advance of the 10-digit telephone number so that the call screening and number interpretation m a switch wouldn’t get confused. This met with some resistance, but ultimately customers got used to the idea. The first two locations to use this revised numbering plan were Los Angeles and New York City back around early 1971.