It is the latest, and by far the largest, of a series of data-center deals that have swept through Northern Virginia. An affiliate of Kimball Small Properties, Dataport is also building a super-hub in San Jose and considering six sites in the New York metro area for a third.

Data centers are windowless, high-security, one-story flex or industrial buildings. They house computers and telecommunications equipment that host Web services, house application-service providers and route Internet and data telecommunications. Typically minded by a skeleton staff, they have been known to cost hundreds of dollars per square foot to equip, far more than even the most richly appointed office buildings. One center in Fairfax, VA cost more than $700 per sf.

Technology and telecommunications companies want to build data centers in the area stretching roughly from Reston and Sterling to Manassas because it is near Dulles International Airport, which links to Washington via the Dulles Toll Road. America Online's headquarters is in Sterling and MCI/WorldCom's is under construction less than a mile away. Most important, all four major providers are laying the miles of fiber-optic cable needed to carry voice, data and video transmissions. US Dataport's project would be close to the new facility of Covad Communication and America Online's $500-million complex.