Cisco NETACAD Routing and Switching v6.0 – Chapter 10

welcome to the Cisco netacad CCNA
introduction to networks video series by Jason Johnson this video is chapter ten
the application layer the material in this video covers the 6.0 version of the
cisco netacad CCNA introduction to networks course I want to thank you for
watching my videos your time is appreciated if you find the material
helpful you can subscribe to my channel and remember to click the notification
button if you want to see when I post new content if you have any questions
you can leave a comment below if you watch to the end of the video I will
have links to the next chapter so let’s take a look at chapter 10 in chapter 10
we’re going to look at application layer protocols explain how the functions of
the application layer the session layer and the presentation layer work together
to provide network services explain how common application layer protocols
interact with end-user applications and we’re going to look at well known
application layer protocols and services and a web applications email protocols
IP addressing protocols how they operate and explain how file transfer file
transfer protocols operate so that’s and then we’re going to have it we’ll have a
summary at the end so let’s take a look at application layer protocols first the
application layer is closest to the end user it’s at the top of the OSI layer
it’s your it’s the application layer protocols help exchange data between
programs running on the source and the destination host the tcp/ip application
layer performs the functions of the upper three layers of the OSI model if
you remember back to a previous video that I did when we did a compare of the
OSI and the TCP application are the tcp/ip the tcp the top three layers of
the i tcp/ip are simplified in to the our I’m sorry the top layer of the
tcp/ip performs the same functions of the upper three layers of the OSI model
common application layer protocols include HTTP FTP TFTP and DNS the
presentation and session layer they format the data compress and encrypt the
data and then pass it on down common standards for video include QuickTime
motion picture expert sub-group our MPEG common graphic
in a image formats such as gif JPEG or PNG or on their the session layer
creates and maintains dialogues between the source and destination applications
and the session layer handles the exchange of information to initiate
dialogues keep them active and to restart sessions that are disrupted or
idle now the tcp/ip application layer protocols they specify the format and
control information necessary for common Internet functions they must be
implemented in both the source and the destination devices and the protocols
implemented on the source and destination hosts must be compatible to
allow communication so this is a graphical representation of the
application layer you have a name System DNS host config email file transfer and
web web protocols there HTTP and HTTPS now how do applications and protocols
interact with the end-user applications the client-server model the client
requests information while servers provide it client and server processes
are considered to be in the application layer and the contents of the data
exchange will depend on the application in use and so email is an example of a
client-server interaction so an email client will send an e to send
information the servers and send said hey I got your email and hey I’ve got
email for you now with peer-to-peer networks data is accessed without the
use of a dedicated server two or more computers can be connected
to a peer-to-peer network to share resources every connected end device up
here can function as both server and a client think of I’ve you’ve got two
computer setup here we’ve got a print client file server here and a print
server file client over here we don’t actually have a print server but this
printer here says hey I’ve got a printer attached to me and if you want to print
to me you can’t so this peer over here says hey I’m sending information for a
print file it has the printer spool it receives the information and then pass
it on to the printer and you get your paper coop just like that so the roles
of the client server are set on a per request basis so you don’t need a server
involved in the peer-to-peer now peer-to-peer applications they can
also use a hybrid system where some resources sharing is decentralized so
you may have a server that has like header file information I’m not gonna
mention any in particular here if you’ve done anything on the Internet you
probably know what I’m referring to as far as as far as file sharing and things
like that a peer-to-peer application so like header file information will be
restored on a server and then I the actual data then will be decentralized
out to a whole bunch of different clients so indexes that point to the
resource locations are stored in the central central directory so it doesn’t
take much overhead at all and in the hybrid system each peer access is an
index server to get the location of all the stored information so it may be
spread out over you know 500 different clients and so I’m looking for a
particular file and I go to the server and I say ok where’s the file index and
it says oh it’s on these 10 different different ten different clients over
here and then my computer then sends requests to those 10 different computers
Act asking for all the file pieces to be sent to me and then it reestablishes it
on my end so that’s a peer-to-peer and decentralized or what they call the
hybrid now common peer-to-peer networks include you know we’ve got edonkey g2
bit tart these may be a little bit out date this is up I’m recording this in
September of 2017 so by the time you’re watching this it might be a little bit
outdated but BitTorrent is one that’s kind of common whore has been common
many p2p applications allow users to share pieces of mini files with each
other at the same time so if you’re a hosting if you’re a hosts peer-to-peer
you may have partial files on your computer so you may not let’s say you’re
you’re sharing a video on your computer you’ve got video information you may
only have a segment of that or a small piece of that video on your computer or
you could be sharing the whole thing but then clients only pull pieces of it from
your so they don’t pull the whole thing from you so a small torrent file
contains information about the location of all the other users and tracker
computers and trackers are computers keeping track of all the files hosted by
the users and for example if you were hosting a you know let’s say a movie on
your computer your information gets sent to those tracker computers so that’s
called BitTorrent there’s lots of them out there utorrent frostwire these these
may be old by the time you’re watching this video but there’s a lot of them out
there I will just say be careful when you are
using BitTorrent and your torrenting files especially if you’re doing
programs and things like that because they are known to be you know basically
pirating software pirating music things like that and that can get you into
trouble because depending upon what country you reside in you can get in
trouble you know you could be breaking a law in your country so just be careful
that if you are doing bit torrenting or you’re doing any type of torrenting that
you are observing laws and you know that that’s that should be a consideration
all right so well known application layer protocols and services what are
they they are things like web and email protocols so hypertext Transfer Protocol
hypertext markup language HTML URL is a reference to a web server so URLs and
you are eyes or the names that most people associate with web addresses so
URLs contain the protocol the server name and the requested file name using
DNS or dynamic name service the server name portion of the URL is then
translated to the Associated IP address before the server can be contacted so it
takes the name so for example and I’m gonna put a plug in for my website Jason
e Johnson comm has an IP address associated with it on the DNS servers so
if you plugged in Jason e Johnson comm or Cisco calm it would go to a DNS over
that DNS server said ok hey I’ve got that name
it’s Jason or its Cisco and here’s the IP address and I’m gonna forward you to
that so HTTP and HTTPS the browser sends a get request to the server’s IP address
and ask for the index now it may not it may send it to the name and the DNS
server does the translation so keep that in mind so and then and then it once it
gets the IP address it sends it there the server sends the request to file to
the client the index dot HTML was specified in the URL and contains the
code for the web page and gets loaded and the browser process is the code
formats the page for the browser based on whether you’re getting Java whether
you’re getting CSS code whatever you’re pulling in that’s all gonna get then to
get displayed in your browser just remember that HTTP is not secure
messages can be intercepted so if somebody’s on your network and
doing wire sniffing and they’re watching those packets go through they can
recreate that HTTP traffic and so whatever webpage you’re looking at or
information you’re looking at they can see it too HTTP uses authentication and
encryption to secure the data so you always want to make sure that if you’re
putting in passwords or if you’re putting in sensitive information that
you’re doing HTTPS I always recommend using HTTPS anywhere as a either Chrome
or Firefox browsers you know get those because that way it forces HTTPS if if
it’s if it’s available and the reason you in the reason web and you may say to
yourself well why doesn’t everybody just use HTTPS well it takes extra overhead
so HTTPS does require more bandwidth so web servers and web web people hosting
don’t put HTTPS on everything if they don’t have to because then it doesn’t
take as much bandwidth because again you’re paying for that bandwidth if it
was up to me everything would be encrypted on the internet and you would
be able to you know get anything but that’s again that’s that’s not me I’m
not paying for the bandwidth now email protocols email is a store-and-forward
method of sending storing and retrieving a messages email messages are stored on
mail servers clients communicate with the mail servers to send and receive and
mail servers communicate with other mail servers to transport messages so you may
send let’s say we have a server here and you’re over here you’ll send your email
up here and then the server sits here and it says okay I need to forward this
information to this mail client over here you don’t send it directly to the
client it’s not like that it goes to a server the server sets there there may
be another server over here and it says hey I need to go to this email client
over here then that client pulls from its server if you’re on the same server
it’s the same email server but if you’re on separate servers you know let’s say
that you’re going from you know for for example going from maybe Gmail over to
Yahoo it goes to different servers on there so email relies on three separate
protocols SMTP pop and IMAP I really recommend doing some reading on those
three you want to know the differences of
those you want to know the ports for those those are ones you need to know
the port’s you’re gonna get those on exams SMTP pop and IMAP you want to go
research and look at those ports and you need to know about these you’re gonna
you’re gonna get questions on them on your exam so the SMTP operation message
formats require a message header and body the
header must have a properly formatted receipt recipient email address and a
sender address and an SMTP client sends the email by connecting to a server on
port 25 and the server receives the message and stores it in a local mailbox
and relays the message to another mail server users use clients to retrieve the
messages stored on that server now the pop operation works a little bit
different messages are download from the server to the client and they do not
stay on the server so email clients direct their pop request to mail servers
on port 110 again just memorize those pop allows for email messages to be
downloaded about the clients device and they are removed from the server so you
have to remember that so if a pop server if I pull my email down and I go to
check it from somewhere else they’re already gone they get downloaded each
client so a download messages resides on the device that triggered the download
so that-that’s-that’s deskey to remember all about pop now I’m at protocols is
another protocol for email it allows for messages to messages to be displayed to
the user rather than downloaded so you’re actually seeing it on the server
so something may be like Gmail that if you’re looking at that you’re looking at
those online so the original message resides on the server until it’s
manually manually deleted by the user you don’t actually download that message
to your client well you do if you’re well you do you’re looking at those you
can download them by the way just let me say you could download those like a
mobile device or your desktop but usually like a browser is gonna host
that information it’s gonna pull it up beat on each time so users view copies
of the message and their email client software and support folder hierarchy is
is granted to organize and store email and so when a user decides to delete a
message excuse me when the user decides to delete a message the server
synchronizes that action and then deletes the message from the server now
domain name service or DNS that is IP addresses are not easy to memorize so
you know remembering you know if I’ve got to remember one ninety two dot one
sixty eight dot 10.11 is my website for my for my web page and then I’ve got to
go over to Google and that’s 192.168.1 numbers for people we don’t do that
anymore we have fun we have fun we used to have phone books now we have devices
that remember all that information for us well DNS allowed for us to come up
with common names so for example and I’m gonna use mine again Jason E Johnson com
I can come up with that and then I can tie that to my web page and you only
have to remember my name you only have to remember Cisco Cisco com and I can
put that in and so you don’t need to remember the IP address or I can have
multiple IP addresses because then I can have web servers on multiple locations
so domain DNS makes server addresses more user friendly computers still need
the actual numeric address or IP address before they can communicate so the DNS
server translates that so a DNS message format common records the a the NS a a a
a and MX for purposes of this course you don’t need to get into these different
kinds here you just need to know that they do come in different types of
records DNS servers search its own records first
if you request it so you can have DNS servers locally that pull information
down so it cuts down on traffic out to the Internet the response is then
forwarded to the client the client then says okay here’s my IP address it
establishes you can use IP config slash display DNS cache to look at the cache
on Windows and you can clear that cache as well so if you’re if you’re having a
problem with an IP address or maybe an IP address change for a DNS and you need
and you’re having problem with that you can do an IP config I think it’s flush
DNS if I remember correctly but you can you can look at that in the reading it’s
got that I’m gonna try to say that off the top of my head and I’m probably
probably gonna get it wrong now DNS hierarchy it uses a hierarchical system
if I can pronounce that right I apologize if I don’t pronounce that
property English high hard cool that’s just I can’t say that word it’s a
hierarchy the naming structure is broken down into small manageable zones so you
have com org au Co you might have dot US for the United States you know for for
colleges you’ll you’ll have dot edu for businesses you’ll have got biz so each
DNS server is only responsible for managing name to IP mappings for a small
portion of the DNS structure so it’s kind of decentralized request for zones
not stored in the specific specific DNS server or forwarded to other servers for
translation and then top-level domains represent the type of a domain of the
country of origin so like a.com org dot us the nslookup you can use nslookup to
place dns queries that’s on Windows you can use that and it’s useful for DNS
troubleshooting so if you’ve troubleshooted your IP address and
you’re like I still can’t get this webpage well you can do an nslookup to
try to see okay let me resolve to IP address to the DNS and and see what the
lookup is on that so that’s another tool to be able to use DHCP or don’t dynamic
host configuration protocol computers need networking IP information so if you
put that in statically or manually you don’t need DHCP but if you’re using DHCP
let’s just say that you turn it on the client and I’m just gonna do it in a
simplified way over here so the client says hey I need an IP address so it
sends out a DHCP discover request the network well the DHCP server sitting
there going hey I’ve got I’ve got DCP for ya I’m gonna send an offer to you so
then it’s gonna request and says oh okay I saw that let me send that and then
it’s gonna send an acknowledgement and it’s gonna say here’s your IP address
and you can use that for a certain set of time and so when you set up your DHCP
server you set that time and then over time that DHCP client says hey I’ve got
this IP address is it okay if I keep using this and if the DHCP server is
still available it acknowledges and say hey yeah you keep using that and we’re
gonna go ahead and renew your DHCP and then DHCP supports both v4 and v6 all
rhyme sorry TCP supports before and dhcpv6 supports ipv6 so v6 has its own
DHCP protocol or Dillons II is service so the operation and I just talked about
that you have to discover the request and acknowledgement and then over time
again if the client cannot contact the DHCP server over time it will lose its
IP address and it’ll keep using it until you have a conflict but it’s supposed to
keep you know get its DHCP and hopefully another one comes online but if it
doesn’t if your DCP server goes down and you’ve got clients out there requesting
over time you’re gonna start getting conflicts so then you have to resolve
that now FTP or File Transport File Transfer Protocol I would see sakes at
File Transfer Protocol FTP I’ve been using FTP FTP for a long time even way
back in you know way back in the days on bulletin board system who used FTP it
was developed to allow transfer of files over the network an FTP client is an
application that runs on a client computer used to push and pull data to
an FTP server it requires two connections between the client and the
server one connection for commands and replies and the other connection for the
actual file transfer now again memorize these ports you’re gonna see them on
exams TCP port 21 is the initiation and an establishment of the first connection
and TCP port 20 is for the second connection and for the actual data
transfer so 21 is for the connection 20 is for the data transfer that’s how I
remember it 20 21 and you would think it was
backwards right you would think hey wait a minute welcome 20 isn’t for the
initiation well it’s just not that way 21 is for the connection and 20
for the or for the establishing connection and server control and 20s
for the data transfer now the client can download or pull data from the server or
upload and push data to the server so if you’re working with like a web server
and you need to up you’re uploading new files to the web server for your web
pages you’re in user clients such as FTP you’re gonna push your files up or you
can delete files off the server so they’re not skin anymore that’s that’s
an example of FTP or how you would use it now SMB our server message block is a
client file serving a client server file sharing protocol let me say that slowly
it shares a common format are all SMB messages share a common format Microsoft
products now support TCP protocols to directly support SMB resource sharing
the Mac Linux and UNIX operating systems have their own implementation of SMB and
you’ll read more about SMB in your reading for the cisco netacad on this
the netacad course alright so that brings chapter 3 presentation to a close
we have looked at in chapter 3 explained the operation of the application layer
and providing support to end-user applications and we took a look at some
well-known tcp applications at the TCP application layer protocols and the
services that are operated on those all right so I hope you have a great day or
I hope that I hope this video was helpful for you if it is give my give it
a thumbs up below I appreciate that and I hope you have a great day

Bernard Jenkins


  1. Thank you for your help and taking your time making these videos! Dropped a sub and a like 🙂

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