The classic BitTorrent peer-listening port — first in the 6881-6889 range clients try.
BEP 3, the BitTorrent protocol specification, describes the convention directly: "Common behavior is for a downloader to try to listen on port 6881 and if that port is taken try 6882, then 6883, etc. and give up after 6889." The port carries peer-to-peer connections in the swarm, not tracker traffic, and it is an inbound listener so other peers can reach you. IANA never registered it — the whole 6869-6887 block is Unassigned — and most modern clients now randomise the port or take it from a UPnP mapping, so 6881 mainly turns up on older or default-configured clients.
Expose with care
The port is meant to be reachable, and a working swarm needs it, but an open 6881 publicly advertises that this host runs BitTorrent and is routinely logged by swarm monitors. Expose it deliberately, on a host where that is fine.
$ nc -vz TARGET 6881Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.