When we talk about the way data flows in a network,
there's really three different ways it can flow,
if you're talking about IP version four.
IPv4 allows us to have unicast, multicast,
and broadcast data flows.
First we have unicast,
and unicast is when data travels
from a single source to a single destination.
So if you want to think about this
like me picking up the phone and calling you,
it's a one-way conversation between you and me.
There's only two people who are talking, me and you,
and it's unicast, going from me to you, or from you to me.
Second, we have multicast,
and this allows data transfer from a specific source
to multiple but specific destinations.
For example, maybe I'm in a classroom
with three different people.
I can now talk to all three of them at once,
because I'm talking to the three people
who are sitting in that classroom,
and it's going from me to those three specific people.
Now, broadcast on the other hand,
is when data travels from a single source
to all the sources on a destination network.
If you think about this, if I go and do a radio broadcast,
I might be speaking on the radio
and I don't know who I'm talking to.
I just know that I'm talking to anybody
who happens to tune in and listen.
That's the big difference between unicast,
multicast and broadcast.
Let's look at each of these a little bit closer
by looking at some diagrams.
First we have unicast, and let's say I have unicast,
and I have one server who wants to send messages
to PC one and PC two.
If I put that message in an envelope,
also known as a packet,
I can then send it out, based on its IP address,
from the server to PC one, or from the server to PC two.
Now, when I go into multicast,
I can send just a single message
and it will be directed to whoever I want it to be into
as part of their multicast group.
So my server in this case is sending out a message
to multicast group number one at 239.2.1.3.
When it gets to the switch,
the switch determines who's part of that multicast group,
and in this example, PC one and PC two
both get a copy of that message.
That's the great thing about multicast,
is I don't have to repeat the message twice,
even if it's the same message,
because it's going out and hitting that switch
and getting repeated everyone in that group.
This works very much like when we're doing things
like broadcasting video using a service like a live stream,
and I can send the message once to my Facebook group,
and then Facebook can multicast that out
to all of the users who want it.
Next we have broadcast,
and when you're dealing with a broadcast message,
this is when the server sends it out
and says, hey, switch, everyone is connected to you.
Tell them all about this message.
By doing that, it's going to broadcast that out
to everybody on the network.
So if you're using a standard Class C network,
that would actually go out to 255.255.255.255.
And when it goes out to the switch,
it's going to send it to everybody who's connected to it.
And in this case, that's PCs one, PC two, and PC three.
Now, some students get confused
between multicast and broadcast.
The key distinction there is with broadcast,
everybody is going to get it.
But with multicast, only those who have opted into it
are going to get that message,
and that's the key distinction there.