UNICAST, MULTICAST, BROADCAST

When traffic is passed between hosts on a network, three different transmission mechanisms are possible. These include unicasts, multicasts, and broadcasts.

Unicasts

A unicast is the most simple network transmission. As the name suggests, it is a direct transmission from one system to one other system only. As such, the destination address will always uniquely identify a single host for whom the data is meant. In a shared Ethernet environment (where a system might be exposed to all frames), systems would check to see whether the destination MAC address matched their own. If it did, it would process the frame. If not, it would discard the frame. On an IP-based network, the address 192.168.1.24 represents a unicast address.

Multicasts

Unlike unicasts, which are meant for a single host, a multicast is meant for a group of systems. Think of multicasts as a one-to-many transmission method. Multicasts are generally used when traffic such as video needs to be passed to many hosts at the same time. In this way, a sender would transmit a single stream of data, which would in turn be picked up by many different hosts. On IP networks, a special group of addresses is reserved for multicasting, those in the Class D range. When multiple hosts need to receive a multicast, they are all configured with an identical multicast IP address. When they receive traffic destined for this shared address, they process it. Do not confuse a multicast address with a regular IP address. In this example, all systems still have a unique IP address, but also “listen in” on a configured multicast address.

Broadcasts

The final type of network transmission is a broadcast. Quite simply, a broadcast is a transmission destined for all hosts. A special destination address designates a broadcast – in Ethernet, the broadcast address is FF-FF-FF-FF-FF-FF. When a host sees frames with this destination MAC address, it knows it has to process the frames. While excessive broadcasts on a network are generally undesirable, many network services depend on this type of transmission.