Alexandria is located six miles below Washington, D.C. on the west bank of the Potomac River. It is bounded on the north by Arlington County, and on the south by Fairfax County. Alexandria is an independent city in the Commonwealth of Virginia has a population of over 120,000. It is largely populated by professionals working in the federal civil service, the U.S. military, or for one of the many private companies that contract to provide services to the federal government. One of Alexandria's largest employers is the U.S. Department of Defense. Others include the Institute for Defense Analyses and the Center for Naval Analyses and the United States Patent and Trademark Office. Alexandria is one of the first cities to offer free wireless Internet access and is home to numerous associations, charities, and non-profit organizations. Alexandria is located just south of Ronald Reagan Washington National Airport. Alexandria Union Station is the city's historic train station and is directly adjacent to the King Street Metrorail station. Three other Metrorail stations are within the city limits. Old Town is chiefly known for its historic town houses, art galleries, antique shops, and restaurants. It is laid out on a grid plan of substantial square blocks and it is a major draw for tourists with many high-income and high-priced neighborhoods. The addressing system in Alexandria is not uniform and reflects the consolidation of several originally separate communities into a single city. Market Square in Old Town is the oldest continuously operating marketplace in the United States. The area to the northwest of Old Town is popularly known as Del Ray. It originated in early 1894 and originally contained six east-west streets and five north south streets that were identical in width, except Mt. Vernon Avenue, which was approximately twenty feet wider. The area has future development plans.