Under North American Numbering Plan (NANP) area codes 630 and 331 are interrelated and serve the cover portions of Chicago’s rural and urban areas. The major areas covered by them are the DuPage County, Illinois. In the northwestern region, area codes 630 and 331 covers neighbouring portion of Cook County which includes the rural areas of Streamwood and Hanover Park. In the western area, Kane County is divided in between the area codes of 847 and 224 in the north, including Elgin, and area codes 630 and 331 in the south, including Aurora. In the southern area, it covers the Will County and a small area of southern Cook County which also includes the rural reigns of Burr Ridge and parts of the village of Lemont, are also included in the 630 and 331 area codes. To the southwest, the city of Yorkville, in exurban Kendall County, is included, as well.

On October 7, 2007, when area code 331 began overlaying area code 630, ten-digit dialing was implemented in both area codes. In 1996, area code 630 had split off from area code 708 and in 1989; area code 708 had split off from area code 312.

The cities, towns, districts and other important places which are there in this area code are as follows:

Addison, Argonnem, Aurora, Bartlett, Batavia, Bensenville, Big rock, Bloomingdale, Bolingbrook, Boulder hill, Bradford group, Bristol, Burr ridge, Burr ridge il, Burr ridgex, Burridge, Carol stream, Citicorp, Clarendon hills, Cloverdale, Darien, Downers grove, Elburn, Electronic data systems, Elmhurst, Eola, Flowerfield, Fox valley, Fox , alley facility, Geneva, ,Glen Ellyn ,Glendale heights ,Glendale hts ,Great lakes area office, Hanover park, Hinsdale, Household finance corp, Household financial services, Inktel marketing, Itasca, Kaneville, Keeneyville, Lafox, Lemont, Lincolnwood, Lions international, Lisle, Little rock, Lombard, Medinah, Millbrook , Montgomery, Mooseheart, Naperville, North aurora, North suburban,Northern ill gas co, ,Northwoods, Oak brook, Oak brook mall branch, Oakbrook ter, Oakbrook terrace, Ontarioville, Oswego, Plano, Plattville, Rates and classification, Roselle, Saint Charles, St charles, Streamwood, Sugar grove, Valley view, Villa park,Virgil, Warrenville, Wasco, Wayne, West Chicago, Westmont, Wheaton, Willow brk, Willow Brook, Willowbrook, Winfield, Wood dale, Woodridge and Yorkville.