Many applications use IP multicasting primarily for two distinct functions.
The first one is to provide a delivery mechanism for multiple destinations. Many applications provide information to multiple recipients simultaneously, commonly used for things like delivering mail, news or updates. Other applications like conferencing and distributing video feeds to an audience.
Without multicasting, these applications would have to deliver a separate copy to each individual destination. Although TCP provides some reliability the resources required without multicasting can be huge. Imagine the overhead a video distribution service or teleconferencing application would generate delivering an individual, yet identical copy of a video stream to multiple recipients in the same network.
Applications which use multicasting can deliver the content once to hundreds of connections. IF you have limited bandwidth this can make a real difference to performance. Many companies use multicasting to deliver corporate announcements across the world to remote employees. These individuals may already be operating under limited network bandwidth perhaps due to local infrastructure or maybe they’re simply using a UK VPN or proxy like this to connect back to the host securely.
The other major use of IP multicasting is to identify the location of servers for some specific purpose. Take for example a network workstation without a hard disk, this client needs to locate a bootstrap server before it can communicate properly on a network. The usual method to locate this server without any prior configuration is to broadcast across all networks until one is located. However a much more efficient method is to use a multicast option. This allows the bootp server to be located without imposing a huge overhead on different hosts which simply don’t provide this service.
To make all this work, multicast addresses need to be used. A multicast address is the combination of high order 4 bits of 1110 and the multicast group ID. These addresses are normally written as dotted decimal numbers usually in the range 126.96.36.199 to 188.8.131.52.