So guys, next we will going to discuss the Ethernet header format.
So in this diagram you can see that the exact Ethernet frame format looks somewhat like this.
So you can see that the first eight bytes are actually a preamble.
So preamble is nothing, but it is a well defined format of eight bits.
Whenever a machine detects this particular format of bits, the machine knows that it is a start of
a new frame.
So preamble is actually a well defined format of eight bits and it is used to mark the start of a new
frame.
The destination address is a six byte destination.
Mac address and source.
Mac address is again a six byte Mac address.
So Destination Mac and Source Mac both are six bytes each.
Then the next field in the ethernet header format is a type field.
So type field helps to identify that.
What is the next layer protocol running on the top of data link layer protocol.
For example, if the type value is zero at zero zero, it means that the next protocol running on the
top of data link layer protocol is IP protocol and therefore the next header of the packet will be IP
header.
Similarly, if the type field is 0041, it means that the next protocol running on the top of data link
layer protocol is IPv6 protocol and therefore the next header after the Ethernet header of the packet
will be IPv6 header.
Similarly, if the type value is 0806, so 806 value is reserved for ARP protocol and if the type value
is 0806, it means that the next header after the ethernet header in the packet is an ARP header.
So Infopath is just a payload data.
The minimum value of this payload should be 46 bytes and the maximum value of this payload can go up
to 500 bytes.
Now, where from this 46 bytes and 1500 bytes came.
For that, you need to open your standard book and learn csma CD that is carrier sense Multiple access
with collision detection protocol.
I'm excluding the discussion of this protocol from this course, because this because the knowledge
of this protocol is not actually very important from interview point of view.
But yes, you should know that.
Why the payload minimum size is 46 bytes and why the payload maximum size is 1500 bytes.
And the last field of the Ethernet frame format is FCS, which is called checksum.
This checksum field is used by the receiving machine to validate if the packet is corrupted or not.
So you can see that the type field is two bytes, whereas FCS field is four bytes.
And the minimum size of the payload section is 46 bytes.
And the maximum size that payload data can grow is 1500 bytes.
Now the question is what is the minimum size of a data link layer frame?
So minimum size is eight plus six plus six plus two plus 46 plus four, and this is equal to 72 bytes.
If you exclude preamble, then this minimum size of Ethernet frame will be 64 bytes.
Similarly, what can be the maximum size of Ethernet frame?
The maximum size could be eight plus six plus six plus.
Two plus 1500 plus four.
So this is equal to 1526 bytes.
And if you exclude preamble, then the maximum Ethernet frame will be 1518 bytes.
So this 1518 bytes is also called M2.
That is maximum transferable unit.
For Ethernet protocol.
The maximum size that a packet can attain is 1318 bytes.
And this 1518 bytes include the size of Ethernet frame itself as well as the entire payload.
So basically what is going out on the wire of the machine is actually called as M2.
So M2 includes all the bits and bytes which are contained in the packet, which is actually shipping
out on the wire of the machine.