Short for Multicast Backbone on the Internet, MBone is an extension to the Internet to support IP multicasting -- two-way transmission of data between multiple sites. The TCP/IP protocol used by the Internet divides messages into packets and sends each packet independently. Packets can travel different routes to their destination, which means that they can arrive in any order and with sizable delays between the first and last packets. In addition, each recipient of the data requires that separate packets be sent from the source to the destination. This works fine for static information, such as text and graphics, but it doesn't work well for real-time audio and video.
With Mbone, a single packet can have multiple destinations and isn't split up until the last possible moment. This means that it can pass through several routers before it needs to be divided to reach its final destinations. This leads to much more efficient transmission and also ensures that packets reach multiple destinations at roughly the same time.
The MBone is an experiment to upgrade the Internet to handle live multimedia messages. MBone servers have special Class D IP addresses. As of March 1997, there were more than 3,000 MBone servers on the Internet.