WEBVTT
Kind: captions
Language: en

1
00:00:00.000 --> 00:00:07.878
Welcome to the first lesson of this free CompTIA Network+ course. The N10-009 exam has five domains,

2
00:00:07.878 --> 00:00:14.259
and we start with Domain 1, Networking Concepts, which is 23 percent of the exam.

3
00:00:14.259 --> 00:00:20.955
Objective 1.1 asks you to explain the OSI reference model. By the end of this lesson,

4
00:00:20.955 --> 00:00:28.282
you'll know all seven layers, what each one does, and how data is wrapped and unwrapped as it

5
00:00:28.282 --> 00:00:29.700
crosses a network.

6
00:00:30.100 --> 00:00:37.668
The OSI model, short for Open Systems Interconnection, describes networking as seven layers stacked

7
00:00:37.668 --> 00:00:43.401
on top of each other. Each layer has one job and serves the layer above it.

8
00:00:43.401 --> 00:00:49.821
That gives everyone a common language: a vendor can say its switch works at Layer 2,

9
00:00:49.821 --> 00:00:56.854
and every network engineer knows what that means. It also gives you a troubleshooting order:

10
00:00:56.854 --> 00:00:59.300
start at the bottom and work up.

11
00:00:59.700 --> 00:01:05.186
Layers are numbered from the bottom. Layer 1 is Physical, then Data link,

12
00:01:05.186 --> 00:01:11.198
Network, Transport, Session, Presentation, and Layer 7, Application, at the top.

13
00:01:11.198 --> 00:01:18.112
A common way to remember them from the bottom up is: Please Do Not Throw Sausage Pizza Away.

14
00:01:18.112 --> 00:01:24.800
The exam uses both the numbers and the names, so learn them together, in both directions.

15
00:01:25.200 --> 00:01:32.498
Layer 1, the Physical layer, moves raw bits: ones and zeros, sent as electrical signals on copper,

16
00:01:32.498 --> 00:01:38.530
pulses of light in fiber, or radio waves for Wi-Fi. It covers cables, connectors,

17
00:01:38.530 --> 00:01:44.487
pins and signal timing. Layer 1 devices pass signals along without reading them:

18
00:01:44.487 --> 00:01:51.636
hubs, repeaters and media converters. In a small office, a damaged patch cable at someone's desk

19
00:01:51.636 --> 00:01:53.200
is a Layer 1 problem.

20
00:01:53.600 --> 00:02:00.032
Layer 2, the Data link layer, delivers frames between devices on the same local network.

21
00:02:00.032 --> 00:02:05.369
It uses MAC addresses, the hardware addresses of network interface cards,

22
00:02:05.369 --> 00:02:10.778
and it checks each frame for errors when it arrives. It has two sublayers:

23
00:02:10.778 --> 00:02:17.868
media access control and logical link control. Switches work here: a switch reads the destination

24
00:02:17.868 --> 00:02:22.400
MAC address and forwards the frame only out of the right port.

25
00:02:22.800 --> 00:02:29.600
Layer 3, the Network layer, moves packets between different networks. It uses logical addresses,

26
00:02:29.600 --> 00:02:35.408
called IP addresses, and it picks a path for each packet, which is called routing.

27
00:02:35.408 --> 00:02:42.067
Routers work at Layer 3, and so do multilayer switches. When the office laptop opens an online

28
00:02:42.067 --> 00:02:48.442
store's website, routers forward its packets from the office network, across the internet,

29
00:02:48.442 --> 00:02:50.000
to the store's server.

30
00:02:50.400 --> 00:02:55.570
Layer 4, the Transport layer, carries data from one application to another.

31
00:02:55.570 --> 00:03:01.498
Port numbers identify the application, like 443 for HTTPS. TCP is connection-oriented:

32
00:03:01.498 --> 00:03:06.461
it sets up a connection with a three-way handshake, keeps data in order,

33
00:03:06.461 --> 00:03:11.562
and resends anything lost. Its unit is the segment. UDP is connectionless:

34
00:03:11.562 --> 00:03:17.283
it sends datagrams with less overhead and no delivery guarantee, which suits voice,

35
00:03:17.283 --> 00:03:18.800
video and DNS lookups.

36
00:03:19.200 --> 00:03:24.307
The top three layers are closest to the user. Layer 5, Session, sets up,

37
00:03:24.307 --> 00:03:29.911
manages and ends the conversations between applications. Layer 6, Presentation,

38
00:03:29.911 --> 00:03:35.161
formats data so both sides understand it, and can encrypt and compress it.

39
00:03:35.161 --> 00:03:41.119
Layer 7, Application, provides the network services that programs use, such as HTTP,

40
00:03:41.119 --> 00:03:46.156
DNS and email protocols. Careful: the web browser itself isn't Layer 7.

41
00:03:46.156 --> 00:03:48.000
The protocols it uses are.

42
00:03:48.400 --> 00:03:54.676
Now let's follow the data down the stack. The browser's request starts as data.

43
00:03:54.676 --> 00:04:02.064
Layer 4 adds a TCP header with port numbers, which makes a segment. Layer 3 adds an IP header

44
00:04:02.064 --> 00:04:09.611
with IP addresses, which makes a packet. Layer 2 adds a header with MAC addresses and a trailer

45
00:04:09.611 --> 00:04:16.999
for error checking, which makes a frame. Layer 1 sends it as bits. The server reverses all of

46
00:04:16.999 --> 00:04:19.700
this. That's called decapsulation.

47
00:04:20.100 --> 00:04:27.261
On the exam, map each clue to its layer. MAC addresses, switches and frames mean Layer 2.

48
00:04:27.261 --> 00:04:34.261
IP addresses, routers and packets mean Layer 3. Port numbers, TCP and UDP mean Layer 4.

49
00:04:34.261 --> 00:04:41.020
Cables, hubs and bits mean Layer 1. Encryption and data formatting point to Layer 6,

50
00:04:41.020 --> 00:04:48.745
and protocols such as HTTP and DNS point to Layer 7. Expect scenario questions that ask at which

51
00:04:48.745 --> 00:04:51.400
layer a device or a problem sits.

52
00:04:51.800 --> 00:04:59.171
To recap: the OSI model has seven layers, from Physical at the bottom to Application at the top.

53
00:04:59.171 --> 00:05:06.389
Bits, frames, packets and segments are the units at Layers 1 to 4, with MAC addresses at Layer

54
00:05:06.389 --> 00:05:12.993
2, IP addresses at Layer 3 and port numbers at Layer 4. Sending wraps data in headers,

55
00:05:12.993 --> 00:05:18.521
and receiving unwraps it. Next, we'll look at the networking appliances,

56
00:05:18.521 --> 00:05:21.900
applications and functions in objective 1.2.