The original City of Alexandria was smaller than today. It extended all the way to Quaker Lane. At the beginning of the last Century, the Virginia created sovereign independent Cities that were not associated with any County. When Alexandria became an independent City, the County of Alexandria became the County of Arlington (Named after Robert E. Lee's former residence The Arlington House at the now Arlington National Cemetery.) In fact the City annexed the Town of Potomac where the Potomac Yards is located.
Needing room to grow, the independent City of Alexandria used its powers to annex parts of Fairfax County until a truce was called between Fairfax County and the City of Alexandria.
Between the continued annexation and the desire for the more prestigious "Alexandria" address, the US Postal Service imposed the 223XX ZIP codes for Fairfax County between the City of Alexandria, and Mount Vernon.
In our REALTOR MLS system called MRIS, the agents and other know when they spot a listing beginning with the prefix FX, we know that it is located in Fairfax County.
Please let me know if I am able to help you with your important real estate questions.
Sincerely yours - Bob Lepelletier - Broker - Associate - Keller Williams Realty - Alexandria/Kingstowne Office (703) 310-7880 or firstname.lastname@example.org