Netflix offers Internet video streaming (“Watch Instantly”) of selected titles to computers running Windows or Mac OS X and to compatible devices. Internet video streaming once came at no additional charge with Netflix’s regular subscription service; however, only a portion of Netflix’s content is available via the “Watch Instantly” option. In its simplest form, video is streamed to the user using standard PC hardware, and requires Microsoft’s Silverlight software to be installed. Viewing is initiated by pressing a “Play Instantly” button, and played back on the PC monitor. Films can be paused or restarted at will. According to a 2011 report by Sandvine, Netflix is the biggest source of North American web traffic, accounting for 24.71% of aggregated traffic.