IP Multicast

With IP Multicast, one single packet sent to a multicast address can be received by many receivers.

IP addresses 224.0.0.0 to 239.255.255.255 are used for multicast. Sending a multicast packet is just like sending a normal unicast IP packet, just send the packet to a multicast address. Only deep in the OS there is a difference: On ethernet, multicast packet must be set to a special MAC address, where the lower 23 bits of the Multicast Address are used as the lower 23 bit of the MAC address.

Receiving multicasts require that the application subscribes the the desired multicast address. This will cause the OS to broadcast a IGMP message which states that the machine will receive traffic on that multicast address.

Most applications of multicast use the connectionless UDP. TCP is a point-to-point protocol, and can not be used with multicast.

Multicasting within a LAN

Multicast is always available in some way within a single LAN.

Dumb ethernet switches usually handle multicasts as broadcasts. So multicasted audio may use up some of the transmission capacity towards other units and PCs on the same LAN. But unwanted multicast traffic will usually be stopped in the network card, and not consume any computer resources. Worst case, simple network cards may forward the multicasts packets to the CPU, but then the OS will drop unwanted multicasts at a very low level.

More advanced ethernet switches may handle multicasts properly by only delivering traffic of a multicast group only to ports where the attached device has signaled that it wants to listen to that group. These are usually Managed Switches. Look for IGMP snooping.

Multicasting across WAN

Across IP routers, multicasting is more problematic. Simple IP routers often does not handle multicasting at all. And even if the routers does handle multicast, advanced configuration of the routers is needed for multicast to work. Special multicast routing protocols are are often needed to manange the dynamic multicast groups.

Multicasting is therefore not generally available at the public Internet, and only organizations with specific requirements implements multicasts in their private WANs.

However, multicast is recently becoming more widely used, driven by the need for efficient distribution of video streams.