What is what area code is 815?

Area code 815 is a telephone area code in the North American Numbering Plan (NANP) for the northern part of Illinois. The code was created in 1947 as a split from area code 312, and it covers cities such as Rockford, Joliet, DeKalb, and Kankakee. In 2007, area code 779 was introduced as an overlay to relieve the demand for telephone numbers in the area.