The move to IPv6 is necessary and unavoidable, but numbers remain plentiful.

On Nov. 10, 1951, when the mayor of Englewood, N.J., made a phone call to his counterpart in Alameda, Calif., the moment marked a turning point in telecommunications. It was the first time a long-distance call had been made in the U.S. by direct dialing, with no operator mediation.

The number dialed – and it was literally dialed, as Englewood’s mayor M. Leslie Downing used a rotary phone – was the first 10-digit phone number to be used since the telephone company AT&T had invented a new coding system for calls.

Stewart SchleyIn creating a direct-dial system, AT&T engineers were determined to replace manually tended circuit boards with a more automated system for connecting calls. In doing so, they faced a big challenge of mathematics. Nobody really knew how many unique numbers the U.S. might come to need, but with household and business phone use growing, AT&T’s team was conscious of the need to future-proof their scheme for allocating numbers.

A key involved slicing the nation into distinct calling areas and assigning a unique code to each. Using only three digits to identify each calling area – part of the objective was to keep direct-dial numbers relatively brief – AT&T determined it would be possible to create an orderly collection of area codes to fulfill the country’s anticipated needs, and still have ample room to add more as additional calling areas were identified and as assigned codes became congested with numbers.

The numbering logic was simple and practical. Each area code would start with a digit from 2 to 9, followed by the number 1 or 0. That way, AT&T could distinguish phone calls directed to the new area codes from an earlier system that aligned calls with local exchanges known by names such as “Gramercy,” where the “G” and “R” corresponded to the numbers 4 and 7. Because telephone keypads had no letters associated with 1 or 0, AT&T’s network could recognize that phone numbers with a 1 or 0 as the second digit were destined for one of the new area codes.

AT&T also introduced some well-reasoned practicality into its numbering system. Knowing it was quicker to dial a 1 or 2 on a rotary phone than, say, a 7 or 9, AT&T assigned the lowest-digit combinations to areas of high telephone concentration. New York City ended up with the venerable 212 area code, while Los Angeles got 213. (For the record, the 1951 call that inaugurated direct long-distance dialing involved area codes 201 for Englewood and 415 for Alameda and the San Francisco Bay area.)

There were 90 area codes in use by the end of 1951. By 2010, the number was approaching 300 as settlement patterns created new population areas and rising phone penetration began to exhaust the available supply of numbers within some existing area codes. Today, Michigan’s 947 area code is the most-used in the U.S., with 92 percent of available numbers taken.

The pressure for more phone numbers has soared since the popularization of mobile phones, which contend for the same pool of numbers as residential landlines and fax and business lines. Even so, the system of 10-digit numbering devised by AT&T has held up remarkably well. Of 1.4 billion phone numbers available to carriers, 640 million remain available, according to a January report by the FCC. The company that administers the North American Numbering Plan now estimates the available pool will last at least until 2031.

Ironically, the same can’t be said for a newer but related system for associating unique numeric codes with telecommunications technologies.

Sometime this year, according to forecasts, the current Internet addressing system will have depleted nearly all of the available addresses used to route data packets to and from their destinations.

The exhaustion of some 4 billion addresses compliant with Internet Protocol version 4 will require a move to IPv6, which relies on 128-bit addressing to create a nearly inexhaustible pool of address combinations. But the migration will be difficult, as millions of devices and gateways built for the older IPv4 won’t recognize IPv6 addresses and data flows. That disconnect will mean costly upgrades for network operators and consumers alike as an entire population of Internet users migrates to a new protocol over time.

The move to IPv6 is necessary and unavoidable. But its very necessity serves as an ironic counterpoint to the fact that 60 years after the creation of the telephone numbering system, numbers remain plentiful. And many rotary phones still work.