Ethernet is the most used layer 2 protocol today and it’s dominance is not likely to end anytime soon. I decided to make a section with some quick facts about Ethernet. There is a lot to know about Ethernet but we usually neglect this because we are very focused on IP. Take a look at an Ethernet frame:
The preamble field is not known to many people. It won’t show up in a packet capture since the network card will already have stripped it before it’s available for capture. So what is the purpose of preamble? The preamble field contains a synchronization pattern that consists of alternating ones and zeros and ends with two consecutive ones. It is used to synchronize node communication but also to indicate where the frame start. Because it is not processed in the same way as the rest of the frame we do not have to count the eight bytes of preamble when calculating Ethernet frame size. This is what preamble looks like:
1010101010101010101010101010101010101010101010101010101010101011