Upon nationwide adoption of area codes in October 1947, Nassau and Suffolk counties, and the lower Hudson Valley, were assigned the 914 area code. In 1951, Nassau and Suffolk were reassigned to the 516 area code.
Despite Long Island's dramatic growth in the second half of the 20th century, this configuration remained in place for five decades. However, by the late 1990s, the proliferation of cell phones and pagers made it obvious that Long Island needed another area code. On November 1, 1999, Nassau retained 516, while Suffolk was reassigned the 631 area code. Permissive dialing of 516 continued until spring 2000.