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What’s a Broadcast?

A broadcast is a network transmission addressed to all devices simultaneously, using the special destination MAC address of all Fs (FF:FF:FF:FF:FF:FF). Any frame sent to this address is forwarded out every port, and every device on the network processes it.

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About this video

A network broadcast is a transmission sent from one device to all other devices on the same network segment simultaneously. Under normal unicast communication, a frame is addressed to a specific destination MAC address, and while all devices on a shared medium may receive it, only the device matching that address processes the frame. A hub makes no distinction and forwards every incoming frame out all ports, but devices still filter traffic based on the destination MAC address in each frame. To intentionally reach every device on a network — including those connected through a switch — a sender uses the reserved broadcast MAC address, written as FF:FF:FF:FF:FF:FF. When a switch receives a frame with this destination address, it forwards the frame out all ports, and every connected device is required to receive and process it. This mechanism is fundamental to many network protocols that need to communicate with all hosts before a specific target address is known.

What you'll learn

What's covered

Network Broadcasts

Aligned to

CompTIA Network+
1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.
Cisco CCNA
1.13 Describe switching concepts
Cisco CCST Networking
4.5 Explain basic switching concepts
Cisco CCST Cybersecurity
2.2 Explain how network addresses impact network security

Key terms

Media Access Control
MAC
A sublayer of the Data Link layer that controls how devices on a network gain access to a medium and transmit data.
Data Link Layer
Layer 2 of the OSI model responsible for node-to-node data transfer and error detection on a single network segment.
Switch
A network device that connects devices within a LAN and forwards traffic based on MAC addresses.
Broadcast Address
The last address in an IP subnet range, formed when all host bits are set to one, used to send a single packet to every device on that network simultaneously.
Frame
The unit of data used at Layer 2 (Data Link layer) of the OSI model, containing source and destination MAC addresses, a payload (the encapsulated Layer 3 packet), and a frame check sequence for error detection. Frames are how switches and other Layer 2 devices think about and forward data.

Transcript

What a Broadcast Is

There are times when your computer wants to address everything on the network. We call it a broadcast.

There are times when a single machine wants to talk to all other machines on the network. If we're working with a hub, then that pretty much sends out all the information to all devices. That is, whatever comes in on one port gets forwarded out all other ports, and all other devices are going to receive that. But not all other devices are going to listen to it. What's going to happen is that if this MAC address is sending to this MAC address right here, all other devices will receive it. However, only this device will really pay attention to it, because it's the only one that it's addressed to.

Furthermore, there are times when we're using a switch, where it doesn't get broadcast out to all devices. But there are times where we want to broadcast out to all devices. So how do we do that?

There is a MAC address that is all Fs. If this device sends a message and the destination is all Fs, the same thing will happen: it'll get broadcast out all ports, and when all these devices see it they will listen and process this frame, since it's addressed to FF FF.

So that is the broadcast address, and that's how a message gets broadcast within this network.

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