VLAN stands for virtual LAN.
It's a technique that lets you have multiple logical LANs
operating on the same physical equipment.
Any frame with a VLAN tag will only be delivered
out of a switch interface configured to relay that specific tag.
This way you can have a single physical network that operates
like it's multiple LANs.
VLANs are usually used to segregate different forms of traffic.
So you might see a company's IP phones operating on one VLAN,
while all desktops operate on another.
After this, you'll find a data payload of an Ethernet frame.
A payload in networking terms is the actual data being transported,
which is everything that isn't a header.
The data payload of a traditional Ethernet frame
can be anywhere from 46 to 1500 bytes long.
This contains all of the data from higher layers such as the IP, transport and
application layers that's actually being transmitted.
Following that data we have what's known as a frame check sequence.
This is a 4-byte or 32-bit number that represents a checksum value for
the entire frame.
This checksum value is calculated by performing
what's known as a cyclical redundancy check against the frame.
A cyclical redundancy check or CRC, is an important concept for
data integrity and is used all over computing, not just network transmissions.
A CRC is basically a mathematical transformation that uses polynomial
division to create a number that represents a larger set of data.
Anytime you perform a CRC against a set of data,
you should end up with the same checksum number.
The reason it's included in the Ethernet frame is so
that the receiving network interface can infer if it received uncorrupted data.
When a device gets ready to send an Internet frame,
it collects all the information we just covered, like the destination and
originating MAC addresses, the data payload and so on.
Then it performs a CRC against that data and attaches the resulting checksum number
as the frame check sequence at the end of the frame.