<p>I'm a newbie at networking.  I understand the concept of multicast, but was wondering if it's reliable on the open/public internet?</p>

<p>It seems like sort of an edge case that different backbones or ISPs might intentionally break to reduce router load or generally segment the network for practical high-use applications.</p>

<p>Is my fear reasonable?</p>

<p>P.S. Follow-up question here:
<a href="https://stackoverflow.com/questions/256125/best-tutorial-for-application-multicasting">Best tutorial for application multicasting?</a></p>

IP multicast in the internet doesn't really work. From the
Wikipedia article
:
For both these reasons, and also
  reasons of economics, IP Multicast is
  not in general use in the commercial
  Internet.
People who desire to participate in the global IP multicast need to attach themselves to the MBONE. The Wikipedia article points out that application-level multicast infrastructures do work, e.g. IRC.