If you're interested in how the shape of California's counties, including Los Angeles, have changed over time, we recommend the Atlas of Historical County Boundaries.
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When there's a risk of an area code running out of phone numbers, an 'Overlay Area Code' is created that has the same geographic boundaries as the existing area code. In this case, the 818 code has been Overlayed with the 747 area code. New phone numbers in the Porter Ranch area will be assigned with one of these codes: 818 or 747. As a result, placing a call in the Porter Ranch area will require 10-digit dialing (where you enter both the area code and then the phone number).