I would have thought everyone using Bittorrent was already aware of this but it appears not.
Most of the ratio implemented torrent sites already blacklist the default port ranges and have been doing so for quite some time.
From the FAQ of one such site.
Why do I get a "rejected by tracker - Port xxxx is blacklisted" error?
Your client is reporting to the tracker that it uses one of the default bittorrent ports (6881-6889) or any other common p2p port for incoming connections.
********** does not allow clients to use ports commonly associated with p2p protocols. The reason for this is that it is a common practice for ISPs to throttle those ports (that is, limit the bandwidth, hence the speed).
The blocked ports list include, but is not necessarily limited to, the following:
Direct Connect 411 - 413
Gnutella 6346 - 6347
BitTorrent 6881 - 6889
In order to use use our tracker you must configure your client to use any port range that does not contain those ports (a range within the region 49152 through 65535 is preferable, cf. IANA).
The size of the range you choose should take this into account (typically less than 10 ports wide. There is no benefit whatsoever in choosing a wide range, and there are possible security implications).
These ports are used for connections between peers, not client to tracker. Therefore this change will not interfere with your ability to use other trackers (in fact it should increase your speed with torrents from any tracker, not just ours). Your client will also still be able to connect to peers that are using the standard ports. If your client does not allow custom ports to be used, you will have to switch to one that does.
Do not ask us, or in the forums, which ports you should choose. The more random the choice is the harder it will be for ISPs to catch on to us and start limiting speeds on the ports we use. If we simply define another range ISPs will start throttling that range also.