Unicast, Multicast, and Broadcast addresses Explained

This tutorial explains unicast, multicast, and broadcast addresses in detail. Learn what the unicast, multicast, and broadcast addresses are and how they work in computer networks.

Communication is a process of sharing or exchanging information. There are three types of communication: one to one, one to many, and one to all. In computer networking, these types are known as unicast, multicast, and broadcast, respectively.

In unicast communication, a device communicates or shares data with one device only. In multicast communication, a device communicates or shares data with multiple devices. In broadcast communication, a device communicates or shares data with all devices.

Each communication involves at least two devices. One device provides the information and another receives the information. At a time only one device can provide or share information. But the shared or provided information can be received by a single device, a group of devices, or all devices in the network.

To provide or receive information, devices use unique addresses. An address can represent a single device, a group of devices, or all devices in the network. Depending on how many devices an address represents, it can be classified in three types: unicast addresses, multicast addresses, and broadcast addresses.

A unicast address represents a single device in the network. A multicast address represents a group of devices in the network. A broadcast address represents all devices in the network. If a device want to share the information only with a single device, it uses the unicast address of that device. If a device want to share the information in the group, it uses the multicast address of that group. If a device want to share the information to all devices in the network, it use the broadcast address of the group.

Devices send or transmit information in small data packets, so packets can travel through any available network paths. Each data packet contains two addresses: a source address of the device creating the packet and a destination address for the device or devices intended to receive the packet.

Since only one device can create the packet, the source address in the packet always remains a unicast address. However, the destination address in the packet can be a unicast, multicast, or a broadcast address.

The destination address in the data packet helps the other devices or computers connected to the network to figure out whether they need to process the data packet when they receive it or ignore it.

Let's understand it through an example. The following image shows an example network.

example network

Suppose, PC1 wants to send a data packet to PC6. Since the recipient of the packet is only PC6, PC1 sets the unicast address of PC6 in the destination address field of the packet. When this packet reaches all computers, except PC6, all computers discard the packet immediately. PC6 accepts and processes the packet.

When a computer receives a data packet, it checks the destination address of the packet. If it matches to the address of the computer, the computer assumes that the packet is intended for it. If it does not match to the address of the computer, the computer assumes that the packet is not intended for it. A computer only accepts the packets that are intended for it. If packets are not intended for it, it will discard them immediately.

The following image shows how this works in our example.

how unicast addresses works example

Now, suppose PC1 wants to send a data packet in the group. PC6, PC7, PC8, PC9, and PC10 are the member of this group. To send the data packet in this group, PC1 sets the multicast address of the group in the destination address field of the packet. When computers receive this packet, only the computers that belong to the group process the packet, the remaining computers in the network ignore it.

When a computer receives a data packet that contains a multicast address in the destination address field, the computer checks whether the computer is a member of the group that the multicast address represents. The computer processes the packet only if it is a member of the group that the multicast address represents.

The following image shows how this works in our example network.

how multicast addresses work

Now suppose PC1 want to send a data packet to all computers in the network. PC1 sets the broadcast address of the network in the destination address field. When other computers receive this packet, they all process it.

When a computer receives a data packet that contains the broadcast address of the network in the destination address field, the computer process the packet. A packet that contains the broadcast address of the network is intended for all computers of the network.

The following image shows how this works in our example network.

how broadcast addresses work

Broadcasts are mainly used in two situations. First, if a device want to send the same information to every device in the network. With a unicast, the device would have to create a separate data packet for each device on the network. But with a broadcast, the device could accomplish the same thing with a single data packet.

Second, if a device want to discover the unicast address of a device. For example, when you turn on a device, initially it doesn't know any address of any other device on the network. A broadcast can be used to discover the addresses of other devices, since they all will process the packet sent on the broadcast address.

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