Local Peer Discovery Multicast Address for BitTorrent

This article explains the multicast IP addresses and port configurations utilized by the Local Peer Discovery (LPD) protocol in BitTorrent clients. LPD—also frequently referred to as Local Service Discovery (LSD)—enables torrent clients residing on the same local area network (LAN) to detect one another and transfer data at local speeds without relying on external trackers or Internet bandwidth. The sections below outline the exact multicast addresses, port numbers, and core mechanics governing this protocol.

Multicast Addresses and Ports

BitTorrent clients use designated UDP multicast groups to broadcast and listen for local peers. The standardized addresses and port configurations are:

The IPv4 address falls within the Organization-Local Scope (239.192.0.0/14), which ensures that these multicast packets are confined to private networks and not routed across the public Internet.

How Local Peer Discovery Works

When a BitTorrent client has LPD enabled and begins downloading or seeding a torrent, it periodically broadcasts a multicast discovery message:

  1. Multicast Announcement: The client sends an HTTP-like BT-SEARCH payload over UDP to port 6771 at the designated multicast address.
  2. Payload Details: The announcement message includes the info-hash of the active torrent, the client’s listening port, and protocol headers.
  3. Neighbor Detection: Any other BitTorrent client listening on the same subnet receives the multicast packet. If the listening client is participating in the swarm for that same info-hash, it initiates a direct peer connection.
  4. Data Transfer: After establishing a connection, peers transfer blocks directly across the local switch or router, utilizing standard peer-to-peer protocols (TCP or uTP) at full local network bandwidth.

Key Benefits of Local Peer Discovery