Any torrent client at startup generates a 20-byte
peer-id value, which indicates the client version and some random value. Peer-id is transmitted to the tracker and is used when connecting to other clients.
Unfortunately, there have recently been several services in the BitTorrent network that are trying to track and identify users by request of copyright holders and law enforcement agencies. Snooping is possible by IP address and peer-id. To make it more difficult for these services, µTorrent client developers in version 3.3 of alpha (build 27841) implemented a
new peer-id generation system , which will now be generated not every hour the client starts, but every hour. Thus, tracking users will become more difficult.
via
TorrentFreak