*** ETHERNET ***
offset  length  description
0       7       preamble (1010..)
7       1       start (10101011)

offset  length  description
0       6       dst mac
6       6       src mac
12      2       ethertype
14      46-1500 data
end     4       FCS 

*** ETHERNET WITH 802.1Q TAG ***

offset  length  description
0       7       preamble (1010..)
7       1       start (10101011)

offset  length  description
0       6       dst mac
6       6       src mac
12      2       Tag protocol ID (0x8100)
14      3b      Priority for 802.1p (0-7)
        1b      CFI (1 for non-canonical, 0 for canonical)
        12b     VLAN ID (0-4095)
16      2       ethertype
18      46-1500 data
end     4       FCS