Internet-Security.com TCP port 6881 details

TCP port 6881 is primarily used by the BitTorrent protocol for incoming peer connections.
The original BitTorrent (Mainline) client listens on TCP ports 6881–6889 by default, starting with 6881.
Vuze/Azureus commonly defaults to TCP 6881 for peer connections (though it’s configurable).
Many rTorrent/ruTorrent setups and commercial seedboxes open TCP 6881 for torrent peer traffic, following the traditional BitTorrent port range.
Blizzard’s Background Downloader used by World of Warcraft historically used TCP 6881–6999 (including 6881) to distribute game patches via BitTorrent.
Due to ISP throttling historically targeting TCP 6881, many modern BitTorrent clients randomize their listening port, but 6881 remains widely recognized for BitTorrent.

TCP 6881