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When we talk about the way data flows in a network,

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there's really three different ways it can flow,

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if you're talking about IP version four.

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IPv4 allows us to have unicast, multicast,

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and broadcast data flows.

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First we have unicast,

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and unicast is when data travels

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from a single source to a single destination.

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So if you want to think about this

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like me picking up the phone and calling you,

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it's a one-way conversation between you and me.

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There's only two people who are talking, me and you,

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and it's unicast, going from me to you, or from you to me.

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Second, we have multicast,

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and this allows data transfer from a specific source

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to multiple but specific destinations.

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For example, maybe I'm in a classroom

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with three different people.

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I can now talk to all three of them at once,

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because I'm talking to the three people

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who are sitting in that classroom,

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and it's going from me to those three specific people.

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Now, broadcast on the other hand,

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is when data travels from a single source

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to all the sources on a destination network.

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If you think about this, if I go and do a radio broadcast,

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I might be speaking on the radio

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and I don't know who I'm talking to.

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I just know that I'm talking to anybody

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who happens to tune in and listen.

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That's the big difference between unicast,

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multicast and broadcast.

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Let's look at each of these a little bit closer

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by looking at some diagrams.

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First we have unicast, and let's say I have unicast,

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and I have one server who wants to send messages

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to PC one and PC two.

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If I put that message in an envelope,

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also known as a packet,

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I can then send it out, based on its IP address,

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from the server to PC one, or from the server to PC two.

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Now, when I go into multicast,

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I can send just a single message

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and it will be directed to whoever I want it to be into

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as part of their multicast group.

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So my server in this case is sending out a message

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to multicast group number one at 239.2.1.3.

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When it gets to the switch,

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the switch determines who's part of that multicast group,

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and in this example, PC one and PC two

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both get a copy of that message.

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That's the great thing about multicast,

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is I don't have to repeat the message twice,

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even if it's the same message,

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because it's going out and hitting that switch

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and getting repeated everyone in that group.

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This works very much like when we're doing things

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like broadcasting video using a service like a live stream,

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and I can send the message once to my Facebook group,

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and then Facebook can multicast that out

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to all of the users who want it.

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Next we have broadcast,

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and when you're dealing with a broadcast message,

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this is when the server sends it out

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and says, hey, switch, everyone is connected to you.

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Tell them all about this message.

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By doing that, it's going to broadcast that out

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to everybody on the network.

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So if you're using a standard Class C network,

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that would actually go out to 255.255.255.255.

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And when it goes out to the switch,

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it's going to send it to everybody who's connected to it.

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And in this case, that's PCs one, PC two, and PC three.

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Now, some students get confused

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between multicast and broadcast.

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The key distinction there is with broadcast,

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everybody is going to get it.

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But with multicast, only those who have opted into it

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are going to get that message,

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and that's the key distinction there.