Unicast, Multicast, and Broadcast Explained

A communication type defines how a device talks to another device on a network. Unicast, multicast, and broadcast are network communication types. Network devices use them to send data packets across the network. Each communication type uses its own address type. A unicast communication uses a unicast address, a multicast communication uses a multicast address, and a broadcast communication uses a broadcast address. A unicast address represents a single interface. A multicast address represents a group of interfaces in the network. A broadcast address represents all interfaces of the network.

Source and destination addresses

Each data packet that travels through a network needs two addresses: a source and a destination. The source address belongs to the device that creates it. The destination address belongs to the device that receives it. The packet sender sets both addresses. In the source field, it sets its address. In the destination address field, it puts the recipient's address. A recipient can be a single device, a group of devices, or all devices on the network. Based on the destination device type, it uses the relevant address type.

Since the originator of a data packet can be only one device, the source address field of a data packet always contains a unicast address. However, the recipients can be a single device, a group of devices, or all devices on the local network. Because of this, the destination address field of a data packet can contain a unicast, multicast, or broadcast address.

Unicast

Unicast is the most common form of communication. In this information sent by an application, such as a web, email, or chat service, travels directly to a single recipient on that network segment using a dedicated IP address. All devices on a network have a unicast address. You can manually assign it or use a DHCP service. A unicast address represents only one interface on the Ethernet LAN. A data packet with a unicast address in the destination field is intended only for a single network device. The following image shows an example of a unicast.

In this example, PC-A generates a data packet for PC-B and places it on the wire. All PCs examine the destination address of the data packet. Since the data packet contains PC-B's address, PC-B processes the packet. All other PCs ignore it.

Multicast

A multicast is a group of devices. All devices in a multicast group listen to a common multicast address. All devices in the group share a multicast address. If a device sends a data packet to this address, it reaches all devices in the group. Multicast membership is dynamic. Any device can join or leave it. Multicast addresses are predefined. The following image shows an example of a multicast.

In this example, PC-A generates a data packet for PC-B and PC-C. Since the packet is for more than one PC and the destination address field can hold only one address, PC-A uses an address that belongs to both PC-B and PC-C. In other words, it uses the group address of both PCs. Technically, this address is known as a multicast address. All PCs examine the destination address of the data packet. Since the data packet contains the group address of PC-B and PC-C, both accept the packet. All other PCs ignore it.

Broadcast

A broadcast represents all devices of the network. A data packet having a broadcast address in the destination field is for all devices on the network. The following image shows an example of a broadcast.

In this example, PC-A generates a data packet for all devices on the network. Since the packet is for all devices, PC-A uses an address that belongs to all devices. All PCs examine the frame's destination address. Since the frame contains the broadcast address, all devices process it.

Conclusion

This tutorial explained network communication types. There are three types of network communication: unicast, multicast, and broadcast. A unicast communication is one-to-one communication. A multicast communication is a one-to-many communication. A broadcast communication is a one-to-group communication.