Windows Server 2008 (codenamed Windows Server Longhorn/Windows Longhorn Server) is one of Microsoft Windows' server line of operating systems. Released on February 27, 2008, it is the successor to Windows Server 2003. Windows Server 2008 is the server counterpart to Windows Vista, and was codenamed Windows Server Longhorn. A second release, named Windows Server 2008 R2 (codenamed Windows Longhorn Server R2/Windows Server Longhorn R2), was released alongside Windows 7 in October 2009. Like Windows Vista and Windows 7, Windows Server 2008 is built on Windows NT 6.x.