- [Narrator] I'll start with the TCP/IP model.
It consists of five layers that provide a set of rules
for network devices to follow.
I like to start at the bottom and work my way up
as this is usually how I troubleshoot.
The physical layer consists of the physical hardware
that makes connections.
This will be cables, their connectors,
and everything related to how light travels
over fiber or electricity moves over a cable.
So when I reference a layer one issue,
that is going to be a physical issue like a bad cable.
The data link layer, layer two, deals with MAC addressing.
It structures its data into frames,
and each frame has a header
that contains various information
like which MAC address it's sourced from,
and which MAC address it's destined to.
As information travels down the model it is encapsulated
and handed to the next layer for processing.
When a router wants to send a packet
it encapsulates it and hands it to layer two
to be delivered to the next device's MAC address.
As a note, as packets move up the model
they are decapsulated.
Switches are usually associated
with transporting information at layer two.
Layer three is the network layer,
which deals with IP addressing.
IP addressing is usually described like the postal system.
IP addressing is a means for me to send a letter
from my house in Texas to my friend in LA,
and have it actually be delivered on the internet.
Just like the addresses of your house
it must be globally unique.
Devices associated with layer three are generally routers.
Routers in essence play hot potato with packets.
Once they know which direction they should go,
they pass them to the person directly next to them
and move on to the next packet.
Layer four is the transport layer,
which deals with Transmission and Control Protocol
and User Datagram Protocol.
TCP and UDP are examined in more detail
in a following chapter.
But a high level, they specify ports
which range from one to 65,535 to transmit information.
Specific port numbers can map to applications
like unencrypted HTTP which uses TCP port 80
or TFTP which uses UDP port 69.
Layer five, the final layer, is application.
This deals with how software running on PCs and servers
communicate with lower layer protocols.
For example, how a browser uses HTTP
to pull information from a web server.
The data structure used to move information
has different names depending on where in the model it is.
At layer two it's known as a frame,
at layer three it's a packet,
and at layer four it's a segment.
There is another model, an arguably even more popular one
known as the Open Systems Interconnection.
Conceptually it's the same as the TCP/IP model
with the bottom four layers matching one for one.
It's the higher layer protocols that happen to be broken out
into additional layers.
The majority of networking configuration and troubleshooting
is done in the bottom four layers.
Understanding an utilizing these models
is important for modern engineers
as they serve as a common language when discussing
where issues may be happening in a network.