Hello Nitin
This is a good point you bring up. If you take a look at the header as described in the Introduction to Ethernet lesson, you will find the following fields:
If you count up all of the green fields, you will indeed get 22 bytes. However, for the purposes of the MTU, the Preamble (7 bytes) and SFD (1 byte) are not counted in the total bytes of the frame. Why? Because strictly speaking, the Preamble and SFD are considered to be part of the Physical layer encapsulation. So really, at Layer 2, which is the Datalink Layer, as far as the MTU calculations are concerned, you only have the following:
So when doing your calculations, you use a value of 14 bytes plus 4 additional bytes if you have an 802.1q tagging involved.
I hope this has been helpful!
Laz