When the North American Numbering Plan was devised by AT&T in October 1947, Nassau and Suffolk counties, and the lower Hudson Valley, were assigned area code 914. In 1951, Nassau and Suffolk were split off as a separate numbering plan area (NPA) with area code 516.
Despite Long Island's dramatic growth in both population and telephone service on Long Island in the second half of the 20th century, this configuration remained in place for five decades. By the late 1990s, the proliferation of cell phones and pagers required further division to provide more telephone numbers for Long Island. On November 1, 1999, Suffolk was split off as a new numbering plan area with area code 631, while Nassau retained 516. Permissive dialing of 516 across Long Island continued until spring 2000.