- [Instructor] When working with addressing
and information flow, I tend to spend
a significant amount of time in Layers 2 and 3,
which are data-link and network respectively.
Layer 2, the data-link layer, refers to communication
between devices on a common switched infrastructure.
This layer is best known for switches and switching.
Each device uses a media access control or MAC address.
MAC addresses are six groups of two hexadecimal digits,
usually separated by colons or hyphens,
but sometimes without a separator.
This layer deals only with switching
so frames don't really care
about the encapsulated IP addresses they are transferring.
Layer 3 refers to sending information via IP addresses.
This layer is best known for routers and routing.
As the information passes from router to router
on its way to a destination, it will use Layer 3 information
to see where to forward the packet,
then repeat the process until it reaches its destination.
As information moves up and down the OSI Model,
some layers have a special name for the data
at one of those layers.
This is called a Protocol Data Unit.
As this information moves down,
it is encapsulated into the lower PDU.
As it arrives to a host and moves up the OSI Model,
it is de-capsulated.
Think of it as Russian nesting dolls,
enclosing one PDU within another.
While I'm focusing on Layer 2 and 3,
all layers are very important
to the troubleshooting process.
Layer 1's PDU is known as a bit.
Since many of networking's issues
can be traced to a physical issue, it must not be forgotten.
Moving to Layer 2, we have frames.
Frames are the last PDUs used
before information is put on the wire for transmission
or the first received off of the wire.
A frame consists of bits arranged as framing bits,
packet payload, and the frame check sequence.
Framing bits are used in conjunction
with a Layer 2 transmission method.
The FCS is an extra bit of code
put in place for error detection.
The PDUs at Layer 3 are packets.
Packets are formed of control information and the payload.
Control information will consist of things
like source and destination IP, sequence information,
and error detection information.
The payload consists of user data to be transported.
Layer 4 consists of TCP segments or UDP datagrams.
The TCP segment consists of a header and a data portion.
The header includes information
like source and destination IP,
sequence number, window size, port numbers, and so on.
The UDP datagram is designed in much the same way,
but the header contains fewer options,
though it does retain the key pieces.
Layer 5 through 7 is just considered data.
It can be compressed, encrypted, or clear text.
Common nomenclature is important when troubleshooting
or simply having a functionality discussion.
Be sure to add the various PDUs to your vernacular.