MulticastRoutes

The theory behind multicast is that instead of having each client in a network request the same info from a server, the server can broadcast the data all at once to a select group of listeners. Individual clients can choose to receive the information being multicasted by joining the multicast group. This is not a broadcast because all systems on the network are not automatically involved.

If your networking hardware, software, and protocols support it, multicast provides an efficient way to distribute many types of information, such as time synchronization (see MulticastNtp).

These instructions are for Linux. Under OpenBSD, you can simply edit /etc/rc.conf and turn on multicast.

When you build your kernel, be sure to enable multicast support under networking options.
You have to have a routing table entry for the multicast route. Here's a sample route command:

/sbin/route add -net 224.0.0.0 netmask 240.0.0.0 dev eth0

It's possible to add this to the RedHat network setup, which has the advantage of preserving the multicast route if you restart networking without rebooting. To do this, edit the file /etc/sysconfig/static-routes and add the line