Common Network Ports – CompTIA A+ 220-1001 – 2.1


One of the first ways
to transfer a file from one device
to the other uses a protocol called FTP, a
File Transfer Protocol. This protocol uses TCP port 20. We call this the
active mode data port, and there is a
TCP port 21 that’s used to control
the communication. TCP does have security
built in, so you can configure a username
and a password that gains access to another system. FTP also supports a mode
called anonymous log in where you can use
the user name anonymous and then any
password you’d like. As it transfers
files, FTP provides what could be called full
featured functionality. You can list the files
available on a system. You can add files, delete,
rename, and provide other file functions as well. If you’ve ever communicated
across the network to another device
at the command line, then you’ve probably used
a console connection that looks very similar to this one. If your console connection
is over an encrypted channel, then it’s probably using SSH or
Secure Shell over TCP port 22. Although this looks very similar
to a console screen you might see if you use
Telnet, Telnet would be over a nonencrypted
channel, but SSH always uses an encrypted
communication link. You may find that some older
equipment doesn’t support SSH and the only way to
communicate to this device and use this terminal
communication is by using Telnet. Telnet stands for
Telecommunication Network, and it uses TCP port 23. Just like with SSH, we
would use Telnet to log in remotely to this
device at the console, but we have to keep in mind that
this entire communication is in the clear. There’s no encrypted
communication. So if you type in your
username and password, anyone capturing those
packets on the network is able to view very
plainly your user name and your password. For that reason,
we don’t commonly see Telnet used on
anyone’s network. And if you need to keep
your system secure, you would probably only use
SSH, instead of using Telnet. In an earlier video, we talked
about mobile devices sending email messages and
the protocol that it used to send those messages
was SMTP or the Simple Mail Transfer Protocol. SMTP can be used to send
messages from a mobile device, or it can be used
to send messages from one server to another. SMTP uses TCP port 25 to
be able to send that data. If you’re receiving
email messages, you’re probably
using POP3 or IMAP. Whenever you’re sending
email, it commonly uses SMTP. If you’re typing a
website into a browser, you’re probably using
the name of the site. So if you type in
www.professormesser.com, behind the scenes,
there needs to be a conversion between
that domain name and the IP address
of my web server that’s where we
would use DNS, which communicates over UDP port 53. This is converting those
names to IP addresses and then back again. For example, if you type in
in www.professormesser.com, that information is sent to a
DNS server, which responds back with an IP address that’s
associated with my web server. We obviously rely
on these DNS servers to be able to provide this
resolution between domain name and IP address. And since we’re using
mostly these domain names and we’re typing
things in at a browser, we’ll probably have
multiple DNS servers. So if we happen to
lose a DNS server or it happens to
become unavailable, we have other DNS servers that
can provide that resolution. If you’re in a web browser and
you’re communicating to a web server, then you’re
probably using HTTP or HTTPS as those protocols. HTTP stands for Hypertext
Transfer Protocol, and HTTPS is the encrypted form
of that or Hypertext Transfer Protocol Secure. These two protocols used
two different port numbers to communicate. The in the clear,
non-encrypted version of HTTP uses TCP port 80. The encrypted communication
occurs with HTTPS and that commonly
uses TCP port 443. If you’re on a mobile
device or desktop computer and you’re receiving
emails, then you’re probably using POP or
IMAP as those protocols. POP3 is the Post Office
Protocol version 3. It uses TCP port 110, and it
provides basic mail transfer functionality. Many of our modern mail
transfers are using IMAP. IMAP is the Internet Message
Access Protocol version 4. It uses TCP port
143 to communicate. IMAP provides some enhanced
features over POP3, such as having multiple
folders and being able to access that email
box from multiple devices. If you’ve ever needed
to view or take control of someone’s
desktop across the network, then you’ve needed to use RDP
or the Remote Desktop Protocol. This uses TCP port 3389 to
provide that remote control functionality. You’ll find that RDP is
available on many different Windows servers and
allows you to either view the entire desktop
of the remote system or view just a single
application that’s running on that remote system. There are many different
clients available to access these remote desktop services. You can run it on a Windows
workstation, Mac OS, Linux, and many others. Microsoft Windows doesn’t
use FTP to transfer files from one system to another. Instead, it uses
its own format to be able to transfer files
called server message block. This is a standard
set of protocols that Windows uses that allows
for file sharing, printer sharing. You might even see it referred
to as CIFS or Common Internet File System. Older Windows systems may
use NetBIOS that is inside of a UDP or TCP packet. UDP port 137 is
NetBIOS name services so that you can find the device
on the network by its name. There’s also UDP port 138,
which is the NetBIOS Datagram service. There’s a TCP
version of this that runs on TCP port 139, which is
the NetBIOS session service. Modern Windows
devices don’t need to parse out these
different NetBIOS protocols and put them inside
of TCP or UDP. Instead, they can communicate
directly over TCP port 445. Just as Windows has its own
protocols for transferring files, Mac OS also has its own
protocols for the Apple Filing Protocol or AFP. These file services in
Mac OS use TCP port 548. To be able to view the
list of available servers, you’re probably going to
use the service location protocol in Mac OS or SLP. The service location protocol
uses TCP port 427 and UDP port 427 to be able to populate a
list of available locations. And very similar
to SMB in Windows, the Apple filing protocol in
Mac OS is also full feature. You have the ability to view
the available list of files to copy files, move files,
rename files, and more. When you turn on your
computer for the first time, it automatically configures
itself with an IP address. It’s able to do
this because it’s using DHCP, which is the Dynamic
Host Configuration Protocol. There is a DHCP server
somewhere on your network, and your client communicates
that server using ports UDP 67 and UDP 68. Once your workstation
receives this IP address, it’s available for a
particular lease time. And before that
lease is up, it has to check back in
with the DHCP server to make sure that it’s still
able to use that IP address. The DHCP servers can
also be configured with DHCP reservations. This means when a
workstation or a server requests an IP
address, the server can recognize the MAC
address of that device and provide the same IP address
to that device every time. If you connect to a corporate
network for the first time, you’re often asked to provide
a username and password. The same thing occurs if
you connect through a VPN or if you log into a web
server that’s on the network. The process of providing
that authentication is usually to a
centralized database, and one very common form of
database that’s used for this is LDAP. This is the Lightweight
Directory Access Protocol, and it uses TCP port 389 to
provide that authentication. This means that you can
store all of your credentials in one single database. And if you ever need to enable,
disable, or make any changes, you simply need to make it
in that centralized location. Network administrators may
have tens or hundreds or even thousands of devices they have
to manage on a single network. In order to constantly
monitor and gather statistics from these devices, these
network administrators use a specialized
protocol called SNMP. This is the Simple Network
Management Protocol, and it uses UDP port
161 to query devices, and it can receive alarms
or traps from those devices over UDP port 162. There may be three
different versions of SNMP that could be running
in an environment. Version one was the original
that provided a non-encrypted, in the clear method
so that a device can communicate to
a router and ask how many bytes have gone
through a particular interface, and that router can respond
back with that value. Version 2 of SNMP
still communicated without any encryption,
but this client could ask many different
questions at the same time and receive a bulk
transfer in response. Many organizations
these days are using SNMP version 3, which
provides message integrity and authentication method. And all of the
information that’s sent between the client and the
remote device is all encrypted.

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