welcome to the Cisco netacad introduction to networks video series by
Jason Johnson this video is module 3 part two of two reference models the
benefits of using a layered model is that complex concepts such as how a
network operates can be difficult to explain and understand so for this
reason a layered model is used two layered models describe network
operations OSI and tcp/ip the OSI has seven layers the tcp/ip has four layers
over here on the left-hand side we have the OSI model you have physical Data
Link Network transport session presentation and application and on the
tcp/ip model side we have network access and you can see that encompasses two the
first two layers of the OSI you have Internet transport and then application
which takes the top three layers of the OSI and combines those now continuing
the benefits of using a layered model there are benefits of using a layered
model in that it assists in a protocol design because protocols that operated
at a specific layer have defined information that they act upon and a
defined interface to the layers above and below if fosters competition between
products from different vendors that can work together it prevents technology or
capability changes in one layer from affecting other layers above and below
and it provides a common language to describe networking functions and
capabilities the OSI reference model is seven layers as I said previously
physical Data Link Network transport session presentation and application I
you will need to memorize this in this course and in networking and beyond and
if you take the networking a plus certification if you take this and I
think it's in the security Plus and certainly in the Cisco certifications
it's in the other certifications and other areas as well you need to memorize
the OSI model for the reference model here it's seven layers how I do it and
it's a new mnemonic that I use please do not throw sausage pizza away
and what I do is when I go into an exam where I need to memorize this
information and recall it I will write down P dnt s P a and then write out the
words and I just remember in my mind please do not throw sausage pizza away
and then I can go through and save physical Data Link Network transport
session presentation application I write all that out and then I can then write
all the details about each one of them that I can remember such as the physical
layer describes the means to activate maintain and deactivate physical
connections this is where our medium operates such as cabling Wi-Fi signals
data like datalink is layer two it describes methods for exchanging data
frames over a common media that's layer two is you'll see a lot of times with
switches layer 3 is the network it provides services to exchange the
individual pieces of data over the network
that's layer 3 usually routers operate at the layer 3 for defined services to
segment transfer and reassemble the data for individual communications 5 provides
services to the presentation layer and to manage data exchange 6 is
presentation it provides for common representation of the data transferred
between application layer services and 7 contains protocols used for process to
process communications next we're going to take a look at the tcp/ip reference
model it is four layers it simplifies a little bit from the OSI model in that it
combines we have four layers here network access internet transport and
application the network access controls the hardware devices a medium internet
determines the best path through the network transport supports communication
between various devices across diverse networks and for application represents
data to the user plus encoding and dialogue control now when we compare
these two as we've seen earlier we have the application on the tcp/ip model
encompasses the layers 5 6 & 7 of the OSI model the OSI model divides the
network access layer and the application layer of the tcp/ip model into multiple
layers the tcp/ip protocol suite
does not specify which protocols to use when transmitting over a physical medium
an OSI layers 1 & 2 discuss the necessary procedures to access the
medium media and a physical means to send data over the network data
encapsulation we have to segment messages on the network to make sure
they go through properly so segmenting is the process of breaking
up messages into smaller units multiplexing is the process of taking
multiple streams of segmented data and interleaving them or winding them
putting them together through the network so those are two terms that
you're gonna need to know for this course the first one is segmenting you
need to know that term and what it stands for and multiplexing is another
term you need to learn so segmenting messages have two primary benefits they
increase speed so large amounts of data can be sent over the network without
typing up it fast forward to their increases speed large amounts of data
can be sent over the network without tying up communications link increases
efficiency only segments which fail to reach the destination need to be
retransmitted not the entire data stream now sequencing is the process of
numbering the sex segments so that the message can be reassembled at the
destination properly TCP is responsible for sequencing the individual segments
so if user one here and user two are both sending messages along the network
they get meshed together but they're all numbered according to their destination
and sender so sender and receiver and they're all numbered based on that and
they can all be put to get back together on the other end now protocol data units
encapsulation is the process where protocols add their information to the
data and each stage of the process a PDU has a different name to reflect its new
functions there's no universal naming convention
for PDUs in this course the PD user named according to the protocols of the
tcp/ip suite so PDUs passing down the stacker as follows data stream segment
packet frame and bits and there's a graphical representation here of that
here's an example of an encapsulation it's a top-down process so the level of
it does it's processes and then passes it down to the next layer and then that
process adds its information to it and as it all the weight gets down to the
layer one and then it sends it across the medium and then on the other side it
D encapsulates it and that's where we have our example here
data is D in caps ladies it moves up the stack so the information is transmitted
across the network it gets on the other side it reads all the ones and zeros and
we're talking about the OSI layer layer one receives it layer 2 and layer 3
passes it on up and then it D encapsulates it and you have your data
stream segment packet frame and received as bits or received as Mansa then framed
segment frame packet segment and data stream data access addresses both the
data link and network layers or you use a dressing to deliver data from source
to destination so the network layer source and destination addresses are
responsible for delivering the IP packet from original source to the final
destination and the data link source and destination addresses are responsible
for delivering the data link frame from one network interface card to another
network interface card so if you are just using NIC cards that's the data
link source and the neck this is our routing this is where we do our
switching area so it's gonna stay on the same local area network and this is
where if the layer it's gonna get routed and sent out and it's gonna use IP
addressing now layer 3 logical addresses the IP packet contains two IP addresses
the source and the destination the source IP address is where it's coming
from the nest ination is where it's going to and these addresses may be on
the same link or remote if they're on the same link if they're on the same
network they just get sent using the layer two but if they are on a remote
network they use the IP addressing and they get routed across some type of
routing device and it uses the IP addressing at that point to send it
where it needs to go an IP address contains two parts a network portion
ipv4 or prefix ipv6 the leftmost part of the address indicates a network group
which the IP address is a member and each LAN or when will have the same
Network portion and host portion ipv4 interface ID the remaining part of the
address identifies a specific device within the group and the portion is
unique for each device on the network so if we look at this here when device
they're on the same network the network on the same network the source and
destination will have the same number on the network portion of the address so
for example your PC one has 192 168 dot 1 an FTP server has 192.168.1 those are
on the same network they just have different host portions but their
network portion is the same so they're on the same network and that's where we
get into subnetting with this with a v4 now the role of the data link layer
addresses is the same IP network so when devices are on the same either net
Network that's the they're on the same local area network the data link frame
will use the actual MAC address or machine address code address of the
destination NIC so these are two terms you need to know Mac and NIC machine a
dress code and network interface card so the MAC addresses are physically
embedded into the Ethernet NIC and our local addressing the source MAC address
will be that of the originator link and it's gonna be a hexadecimal code when
the network interface cards are created the whoever is creating those whoever
whoever is creating those network interface cards will be given a
hexadecimal code to put on there and then a unique code so there's a unique
hexadecimal code to whoever created it and then a unique code within that
manufacturer so the source MAC address will be that of the originator the
destination will be that of where it's being sent to now when we go to over a
mote network that's what happens one more the actual destination is not on
the same LAN we have to do some routing and that's where we use the IP
addressing so when PC one tries to reach the webserver so PC one here tries to
reach a webserver but it's not on the same local area network well it can't
use its MAC address anymore it has to use the IP routing or IP addressing for
routing so then the routers use the IP addresses they send that through to the
web server even though
the MAC address is still there it's only reading it's only using the IP address
the routing now the role of the network layer addresses when the source and
destination have a different network portion that means they're on different
networks so here for 192 168 not 1 and then web
servers 172 dot 16.1 those can't see each other even though their connections
are all connected together they can't see each other because they're on
different portions of the subnet when the final destination is remote layer 3
will provide layer 2 with the local default gateway IP address also known as
the router address the default gateway is the router interface IP address
that's part of the land but it'll be a door or a gateway out of your local area
network and then all devices on the land must be told about this address or the
traffic will be confined to land only if it's not if you're not giving out a
gateway address it won't be able to wrap be routed anywhere so once layer 2 on
the PC one Forge to the default gateway the router then can start the routing
process of getting the information to the actual destination the data link
addressing is local addressing so it'll have a source and destination for each
link and the MAC addressing for the first segment is in this hexadecimal
number these are fake numbers here but it'll it'll look something it'll be
hexadecimal on there and then you and then while the l2 local
addressing will change from the link to link or hop to hop the l3 addressing
remains the same and since datalink addressing is local addressing it will
have a source and destination for each segment or hop of the journey to the
destination so the MAC addressing for the first
segment is source PC one and then sends the frame and the destination is the
router gateway and sends it through the Mac addressing for the second hop is the
source first router exit interface since the frame destination second router
Raffi's receives frame and then the last segment is the second router exit
interface sends the frame and then the webserver NIC
so it changes dates it as it goes along now notice that the packets not modified
but the frame is changed so therefore the L 3 IP addressing does not change
from segment second like the L to Mecca dressing does
they'll three addressing remains the same since its global and its ultimate
destination is still the web server some of the things we learned in this module
or rules protocols must have a sender and a receiver common computer protocols
include message coding formatting and encapsulation size timing and delivery
options protocols they send messages across network to send a message mess
that up to send a message across the network requires the use of several
protocols each network protocol has its own function format and rules for
communications we talked about protocol Suites protocol suite is a group of
interrelated protocols and tcp/ip protocol suite or the protocols used
today we looked at standards organizations and again I encourage you
to go out on and do some research on standards organizations open standards
encourage interoperability competition and innovation we took a look at
reference models the two models used in networking of the tcp/ip and OSI model
the tcp/ip model has four layers and the OSI model has seven layers and you do
need to learn both of those D encapsulation or data encapsulation the
form that a piece of data takes at any layer is called protocol data unit or a
PDU there are five different PDUs used in the data encapsulation data segment
packet frames and bits and I'll almost guarantee you that you're gonna see that
on a exam or a quiz so you want to make sure that you understand that and then
data access the network and data link layers are going to provide addressing
to move data through the network layer 3 provides IP addressing and layer 2 will
provide mac addressing and the way these layers are handled will depend on
whether the source and destination on the same network or if the destination
is on the different network on the source ok this has been module 3 part 2
of 2 I hope it's been helpful for you and I hope you have a great day