WASHINGTON (AP) _ America Online said today it will buy Netscape in a $4.21 billion deal that would effectively conclude Netscape's dramatic rise and fall as an Internet software pioneer. The agreement also includes Sun Microsystems Inc.

The agreement also cements AOL's position as a new superpower in the high-tech world, giving it remarkable reach and enough influence to challenge industry giant Microsoft's dominance in key areas.

``The acquisition of Netscape is a big step forward for America Online that will greatly accelerate our business momentum,'' AOL Chairman Steve Case said. ``Netscape has played a key role in helping consumers benefit from the enormous power of the Internet, and we share the same mission.''

AOL becomes the world's largest Internet provider, the distributor of Netscape's hugely popular browser software and the owner of two of the four most popular sites on the Web.

Netscape's site alone draws 20 million visitors each month. The other three most popular sites are operated by AOL, Microsoft and Yahoo!.