Blizzard Downloader is a BitTorrent client used in Blizzard Entertainment's MMORPG World of Warcraft. It is used to download large updates for the game, smaller updates being downloaded directly from the server. Early versions were based on the source code of the original BitTorrent client, though it is unclear what the source model is now. When a large update is issued a Blizzard Downloader is downloaded to the WoW directory and it fetches the update file, using the standard port 6881. BNUpdate then applies the patch.