Network Solutions Inc., a Herndon-based computer services company, has won the largest contract ever granted to a minority-owned firm by American Telephone & Telegraph Co., the two companies said yesterday.

The four-year, $12 million contract, expected to be officially announced today, calls for Network Solutions to provide engineering and support services for FTS-2000, the huge new federal telecommunications network assembled by AT&T and US Sprint Communications Co.

The system, which through 300,000 miles of fiber optic cable links about 1.3 million federal employees in all 50 states plus Guam, Puerto Rico and the Virgin Islands, allows users to exchange voice, data and video information through their telephones. With a 60 percent share of the network -- the other 40 percent is run by US Sprint -- AT&T has 785,000 federal customers.

"This contract was on our list of 'must wins' this year," said Emmit McHenry, president and chief executive of Network Solutions. "This extends our telecommunications service business and gives us the critical mass to go after larger contracts in commercial markets." McHenry said that the company will hire more employees, but it was too early to say how many.

The deal is a coup for Network Solutions, which has 470 employees in 20 nationwide locations and $48 million in annual revenue. The company was listed this year as the 11th-largest black-owned business in the country in Black Enterprise magazine's annual ranking of the nation's 100 largest black-owned businesses.

The privately held company last fall decided to pursue the contract after attending an AT&T opportunity fair in Vienna. AT&T regularly holds such events as a forum for minority firms to do business with the company. According to AT&T, more than 70 firms competed for the FTS-2000 contract that Network Solutions won.

"This is a very significant event for us," said Dennis Roth, the AT&T vice president overseeing the FTS-2000 project. "We've got a very large number of activities underway to identify minority vendors capable of providing the technical support we need to provide high quality service to our customers.

"Network Solutions has the right technological approach and the requisite previous experience to do a first-class job for us."

Among the services that Network Services will provide are site surveys, installations and maintenance of the FTS-2000 voice and data equipment. The company also will manage and operate an equipment and testing center, through which it will certify as workable all FTS-2000 equipment before it is installed at federal agencies.