What does an ethernet header look like?

Question

What does the ethernet header look like?

Is it:

1|2|3|4|5|6|7|8|9|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|27|28|29|30|31|
..................................destination mac..................................
...................................|...................source mac..................
...................................................................................|
...............type................|

Or:

1|2|3|4|5|6|7|8|9|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|27|28|29|30|31|
..................................destination mac..................................
...................................|...................source mac..................
...................................................................................|
...............type................|.......................data....................
......................................up to 1500...................................|
crc/fcs|

Or is it:

1|2|3|4|5|6|7|8|9|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|27|28|29|30|31|
...Preamble..|................................destination mac......................
...........................................................|source mac.............
...................................................................................
.............|.type........................................|data....................
......................................up to 1500...................................|
crc/fcs|

I found many different opinions on this in the web.

Answer 1

Neither of them. Following the specs the frame looks like:

• Preamble: 8 bytes
• Destination mac: 6 bytes
• Source mac: 6 bytes
• Type/length: 2 bytes
• Data: 46-1500 bytes
• Frame check: 4 bytes

The specs are here: http://standards.ieee.org/about/get/802/802.3.html

A more useful and easily accessible explanation is here: http://wiki.wireshark.org/Ethernet

The header is the frame before the data. The MAC header is point 2-4 (14 bytes). The MAC trailer is 4 bytes (last point). The ethernet frame thus consists of the preamble (8 bytes), the MAC header (14 bytes), data and the trailer (4 bytes).