The U.S. state of Illinois is divided into ten distinct geographic numbering plan areas in the North American Numbering Plan (NANP), which are assigned a total of seventeen area codes. When the 86 original North American area codes where established in 1947, all of the Chicago metropolitan area was located in numbering plan area 312, the rest of northern Illinois was 815, central Illinois was 217, and southern Illinois was 618.
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In 1954, the 217 and 618 numbering plan areas were slightly remapped, and in 1957, area code 815 was split for the assignment of area code 309 to western central Illinois. No further changes took place for 33 years.
| Area code | Year created | Parent NPA | Overlay | Numbering plan area |
|---|---|---|---|---|
| 312 | 1947 | – | 312/872 | Central city area of Chicago, including the Chicago Loop and the Near North Side |
| 773 | 1996 | 312 | 773/872 | Chicago area, exclusive of NPA 312 |
| 872 | 2009 | 312 | 312/872 | City of Chicago |
| 773 | 773/872 | |||
| 708 | 1989 | 312 | 708/464 | South suburbs and inner west suburbs of Chicago, including the Chicago Southland and most west and south suburbs in Cook County including Melrose Park, Norridge, Oak Park, Oak Lawn, Chicago Heights, and Orland Park. |
| 464 | 2022 | 708 | ||
| 630 | 1996 | 708 | 331/630 | West suburbs of Chicago in DuPage County and Kane County including Wheaton, Naperville, and Aurora |
| 331 | 2007 | 630 | ||
| 847 | 1996 | 708 | 224/847 | North and northwest suburbs of Chicago including all of Lake County, part of McHenry County, northern Cook County, and northeastern Kane County. The NPA includes Evanston, Des Plaines, Waukegan, and most of Elgin. |
| 224 | 2002 | 847 | ||
| 815 | 1947 | – | 779/815 | Northern Illinois outside of the immediate Chicago area including Joliet, Kankakee, LaSalle, DeKalb, and Rockford |
| 779 | 2007 | 815 | ||
| 217 | 1947 | – | 217/447 | Central Illinois, including the region running west from the Illinois-Indiana border through Danville, Effingham, Champaign–Urbana, Decatur, Springfield, Quincy until Illinois' western border with Missouri and Iowa |
| 447 | 2021 | 217 | ||
| 309 | 1957 | 217 | 309/861 | Central-Western Illinois including Bloomington–Normal, Peoria, and all the way west to the Illinois part of the Quad Cities including Moline, and Rock Island |
| 815 | ||||
| 861 | 2023 | 309 | ||
| 618 | 1947 | – | 618/730 | Southern Illinois, including Carbondale and most of the Metro East region of St. Louis suburbs in Illinois |
| 1954 | 217 | |||
| 730 | 2023 | 618 | ||
