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One of the unique
challenges with phones

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today is that if we get a
call and we have the same ring

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tone for everybody,
we really don't

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know who is calling until we
take another look at the phone

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and learn more details about it.

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And one solution to that
is to have unique ringtones

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for certain callers.

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And in the world of
networking, a computer

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can have many
services available.

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For example, web server has
HTTP web services available.

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Or a specific type of file
server has files available.

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But when a customer comes
knocking at the door looking

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for various services, how does
that server know exactly which

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services the customer is after?

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And one of the solutions to
that is to use port numbers.

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So as an example of this,
let's use two devices.

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We have PC 1 over here on the
left and a server on the right,

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and they are connected
over a network.

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And for our discussion,
let's imagine

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that this server has several
services, including a Telnet

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service that's running.

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And what Telnet
allows a person to do

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is to connect remotely
to another system

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and get a command
line interface.

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That's the Telnet protocol.

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And if this server has
some software running

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that provides that
Telnet service,

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it is listening for an incoming
request on a specific port.

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And for Telnet, that well-known
and agreed-to port for a Telnet

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service is port number 23.

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So 23 is the well-known
port for a Telnet server

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that's running the
Telnet service.

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Now, at the same
time, the same server

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could be running software
supporting web services,

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or HTTP.

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So in that sense,
we could also say

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the server is providing an
HTTP service or web service.

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And the well-known port for
HTTP is TCP port number 80.

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So the question I have is, if
this client over here on PC 1

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makes a request
over to the server,

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how does the server
know if the PC is

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going for web services
on TCP port 80

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or if it's going for Telnet
services on TCP port 23?

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And the answer is
that the client,

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in its request for
services, is going

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to include the port
number for the service

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that it wants to access.

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So from the client's
perspective,

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let's imagine that we
have, at PC 1, a user,

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like Bob or Sally
or Lois, and they

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have some software running
on their computer that's

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acting as a Telnet client.

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So they're running
that application

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and thus taking advantage
of the protocol stack

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that's going to allow them
to reach out over the network

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and access a Telnet service.

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So as this Telnet
client on PC 1 logically

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is sending this information
down the protocol

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stack on their local computer,
when it gets to layer 4,

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it's going to use a
destination port of 23.

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And by doing so, once that
information is delivered over

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to the server and it works its
way up the protocol stack, when

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it gets to the transport
layer, when it sees

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that well-known port of
23, the Telnet service

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is considered to be listening
or paying attention to port 23,

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and as a result, can go ahead
and process that request,

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that Telnet request,
via the Telnet

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service running on the server.

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And Telnet happens to use--
at the transport layer,

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it happens to use TCP.

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So the destination port
number in the request

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would be as part of the
TCP information, layer 4,

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destined to port 23.

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And on the server side,
the Telnet service

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is listening for TCP port
23 for incoming requests.

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So that would be for the
request portion of our show.

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Now, what happens if we
have a really good server?

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What should it do if
it gets a request?

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And here's what I'm thinking.

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I'm thinking maybe the
server should respond back

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to the client.

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However, if this
server responds back

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to the client, how
in the world do

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we make sure that
the Telnet server

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responds to the correct
port back on the client?

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And the answer begins
back with the client.

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When the client makes
that initial request,

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not only is it going to
include the destination

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port, the well-known port for
a Telnet service on port 23,

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but it's also going to
include, as part of the Layer 4

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information, a source port.

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And that begs the
question-- well,

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what exactly is the source port
going to be used by the client?

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And the answer
depends on what are

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the available unused ports at
that moment on this client,

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on this PC.

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And then it's up to
the PC and the client

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to go ahead and choose
a random unused port.

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So for our example
here, let's imagine

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that the PC made
a free will choice

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and chose the port number
of 1550 as its source port.

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So in the initial request,
at the transport layer,

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this Telnet client is
using a destination port

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at layer 4 of 23 and a source
port that it chose of 1550.

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So when that request comes
in, the Telnet service

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processing that request sees
that the destination port

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is 23 on its local system
and that the source port

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that it's coming from is 1550.

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And as a result, when
this server responds back

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to the client, it's going
to include any TCP layer 4

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information.

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It's going to include
the fact that it's

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going to be destined to
the client's port of 1550.

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So on the server side, it
would use-- for the response,

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it would use 23 as the
source port and port 1550

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as the destination port.

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And that would be for the
response from the server

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back to the client.

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So let's do one more
example just to confirm.

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Imagine that this
server on the right

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is running an HTTP service and
that well-known port for HTTP

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is TCP port number 80.

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And let's imagine that the PC
here is making an HTTP request.

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And let's imagine
that the source port

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that PC 1 is using
for that conversation

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to this server on
port 80 is going

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to be source port number 6783.

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And so given this information
and the discussion

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that we just had regarding
how ports are used,

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I'd like you to pause the
video here for a moment

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and think to yourself, OK, for
the packet, the initial packet

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that's going from PC
1 over to the server,

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what would the source
and destination ports

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be in that initial request
from the client at PC 1?

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And then if the server
responds to that request, what

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would be the server's
source and destination

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ports in the reply packet going
from the server over to PC 1?

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And here we go.

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Well, the PC, the source
port for the initial request,

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is going to be port number 6783.

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It's right there.

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So we'll just put that in.

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And for the destination
port, if it's

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going to the web
services on this server,

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the destination port is going be
that well-known port for HTTP,

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which is port number 80.

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And for the reply coming
back from the server

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back to the client, it would be
these two ports for the reply

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traffic, but reversed.

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The source port would be
coming from the well-known port

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of port 80 on the server back
to that client's initial port

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that it chose of 6783.

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In this Nugget,
we've taken a look

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at how ports can be used at the
transport layer in the TCP/IP

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Protocol Suite.

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I hope this has been
informative for you,

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and I'd like to thank
you for viewing.

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