Original subtitles

In our previous video, we talked about how services have port numbers

that they use so that other devices can communicate and use those services.

We refer to these as well known port numbers, because both the server

and the client need to know exactly what port number they'll use to communicate.

This is not only important for the server and the client,

but it's also important for any firewalls

that may be in the middle of that communication.

Firewalls have to decide whether to allow or disallow this traffic,

and they often make that decision based on the port number.

As we go through this video, you'll see that

there are a lot of different port numbers that you should use,

and if you've never worked

with port numbers before, this will seem like rote memorization.

But the port numbers themselves

become easier and easier to remember as you continue to use them.

It's important to know these port numbers

and what protocols are often used by a particular port number.

But it's also important to know when you would use these protocols.

For example, your exam might ask you what port number is used by an application

that transfers data in a particular way across the network.

Let's start

our conversation of port numbers with FTP or the File Transfer Protocol.

As the name implies,

this is a protocol that transfers files from one device to another.

There are two port numbers used by FTP.

One is TCP port 20.

This is for data transfers and TCP port 21,

which is used to control the data transfer.

FTP is a generic way to transfer data between devices, but it commonly requires

some type of authentication to log in to that remote device.

So you might use a username and password, although some systems will allow you

to log in as anonymous and use any password to gain access.

The FTP protocol

also includes

a number of file management functions, so when you connect to an FTP server,

you can list all of the files on that server.

You can add, delete, rename and perform other file management

functions, all by using FTP.

There may be times when

you need to connect to a remote device through a terminal or command line.

Front end.

It's common to use secure shell or SSH

to provide this terminal connection to a remote device.

SSH commonly communicates over TCP using port 22,

and it has this text based front end to be able to access that remote device.

Although we see plain text on our screen, any communication

sent over the network is sent as encrypted data.

That's where the secure comes from.

In the name Secure Shell.

There's another way to connect

to a remote device using this text based or console front end,

and it's using telnet or the telecommunication network protocol.

Telnet commonly uses TCP port 23 to provide this connection.

Just like SSH, telnet provides this text based front end

that allows us to connect to the remote console of another device.

But unlike SSH, all of the communication between this telnet front end

and the telnet server is all sent in the clear or not encrypted.

This means that anyone who might be capturing packets between

these two devices will see everything that you're sending over this link,

including usernames, passwords, and everything else.

This is why we often say that you shouldn't use telnet

on your production networks,

and instead you should always use SSH for terminal communication.

There are probably millions of email servers

that are located on the internet, and the protocol that those email server

use to communicate with each other is the Simple Mail Transfer Protocol or SMTp.

SMTp commonly uses TCP using port 25.

Not only is SMTp used to communicate between mail servers,

but it's also used for our mail clients to send mail to a mail server.

So if you're using a mail client on your mobile device or your desktop computer

and you're sending an email message, you could be using SMTp.

Although it's common to use SMTp to send mail,

receiving mail is done using a completely different set of protocols.

So if you were to look at your email client, you're probably using iMap or Pop3

to be able to receive mail and SMTp to be able to send mail.

If you need to connect to a remote website, you would probably type

into your browser dub dub, dub dot Professor Messer com and hit enter.

Behind the scenes, your system needs to be able

to communicate with the Professor Messer web server,

but it has no idea what the IP address is for that server

to be able to resolve an IP address from that fully qualified domain name.

Your system will use DNS or the Domain Name System.

DNS commonly uses UDP port 53

to be able to make this connection between you and the DNS server.

These are obviously very critical resources

because we don't often memorize IP addresses of servers, and very often

server IP addresses can change without any type of notification.

It's DNS that provides that resolution between a name

that we're typing in on the browser, and the IP address that will be used for

the actual communication.

If you've ever

used your mobile device on the Wi-Fi network of a coffee shop,

then you've automatically received an IP address that you can use on that network.

This IP address was assigned automatically using

the Dynamic Host Configuration Protocol, or Dhcp.

The well-known ports for Dhcp are UDP

port 67 and UDP port 68.

Of course, you need a Dhcp server to assign these IP addresses

to devices on your network, but that functional is commonly built

into the routers and wireless access points that we use these days.

This Dhcp server will have a large pool of IP addresses

that can then be assigned to devices on your network.

All of these IP addresses are assigned in real time,

so when you start up your computer, it queries the Dhcp server, and the Dhcp

server assigns your device all of your IP configuration details.

Dhcp uses a leasing system to assign these IP addresses,

so you are only using this IP address for certain amount of time.

And after that lease expires and you're no longer on the network,

that IP address goes back in the pool for someone else to use.

Network administrators can also configure Dhcp servers

to always assign the same IP address to certain devices.

So if there are routers, firewalls, switches, and other infrastructure devices

on your network,

your network administrator may configure Dhcp reservations.

So those devices always receive the same IP address every time they're booted up.

This also means that if you need to change any of the IP configurations

on these devices, you don't have to go to the devices

to make those changes, you simply make them on the Dhcp server.

The next time that device requests the Dhcp address, it will receive

the new configuration.

If you've ever used a

web browser, then you've used Http and Https.

Http is the Hypertext Transfer Protocol, and this is

the common protocol used by our browsers to communicate to web servers.

There are two different protocols depending on the type of communication

that you'll be doing.

If you're communicating in the clear or in a non encrypted form.

You would be using the Http protocol which uses TCP and port 80.

If your browser's communicating

over an encrypted connection, then it's probably using Https.

The s being for secure and that uses TCP port 443.

We mentioned earlier that sending email messages

can use SMTp or the Simple Mail Transfer Protocol,

but to receive email messages, you would commonly use Pop3 or iMap.

Pop3 is the Post Office Protocol version three, and it commonly uses TCP port 110.

Pop3 was designed to retrieve email messages

to an email client, but it wasn't built for multiple email clients.

And of course, these days we tend to walk around with many different

mobile devices, all accessing the same email inbox

to be able to synchronize across all of these different mailboxes.

We commonly use iMap for or the Internet Message Access Protocol version four.

iMap commonly uses TCP port 143 to be able to download and manage that mailbox.

Many operating systems have their own method of transferring

files and information between devices using that operating system.

Windows commonly uses server message block or SMB

to be able to provide this connection.

This is the protocol commonly used by Microsoft Windows,

so if you're transferring files between devices

or you're sending a print job to a printer, it's probably using SMB.

You might also hear SMB referenced as CIFs or the Common Internet File System.

SMB uses a number of different protocols to be able to communicate,

and if you're communicating to an older windows

machine, you're probably using net bias over tcp IP.

Net Bios is the network basic input output system.

This uses UDP port 137 as a name service function,

so that it can find devices on your network by the name, and uses

TCP port 139 to set up a session and transfer data between devices.

In most modern versions of windows that Bios isn't used,

devices can communicate directly between each other using TCP IP.

In those cases, it would use a direct SMB

connection using TCP port 445.

This allows us to have a direct SMB

communication between two devices by using only TCP, IP.

If you plan on doing any work as a network administrator,

you'll become very familiar with S&P or the Simple Network Management Protocol.

S&P allows a network management device to query these infrastructure devices

for performance details, and receive those metrics in return.

This uses UDP port 161 to perform these queries.

You can also configure the infrastructure device to monitor

for certain metrics, and if it exceeds any of those metrics,

it can send an alert to the management station.

We refer to these alerts as traps, and the traps use UDP port 162.

If you're configuring SMP on a device, it will ask you

if you're using version one.

Version two, or version three.

Version one was obviously the original SNMp version.

It sends structured information across the network.

But all of this information was sent in the clear or in a non encrypted form.

We introduced a newer version of SMP with us AMP version two,

which allowed us to do bulk transfers of information.

But still all of that information was being sent in a non encrypted form.

To be able to include additional security with us and AMP.

You need to use S&P v3.

This includes encryption that allows us to have message integrity

authentication and encryption of all of the SNMp data.

We use directories extensively on our modern networks.

And one very common protocol to use to query these directories is Ldap.

Ldap is the Lightweight Directory Access Protocol,

and it commonly uses TCP port 389.

There are many implementations of Ldap, but one of the most popular is Microsoft

Active Directory, which allows you to query that Active Directory Server

using the Ldap protocol.

If you've ever worked in a support role or on a helpdesk,

then you've probably performed some type of remote access to someone's desktop.

One very popular protocol to provide this remote communication

is RDP or the Remote Desktop Protocol.

This is the standard protocol used by windows for the remote sharing,

and it commonly uses TCP port 3389.

These remote desktop services are available in many different editions

of windows,

and if you're using windows today, then you probably have built

into the operating system the ability to connect to

or receive a remote desktop session.

RDP can be used to take over and control an entire system,

or just to run a single application from that server.

Although you'll find servers running RDP almost exclusively on windows,

there are clients that you can run on almost any operating system.

So if you're running Linux, Mac OS, Android, or any other operating system,

there's probably an application you can load that will allow you

to connect to a windows device using the Remote Desktop Protocol.