*** IPv4 ***    
offset  length  description
0       1       version(4)=4 / IP Header Length(4)
1       1       type of service (TOS)
2       2       length
4       2       identification
6       2       flags(3) / fragment offset(13)
8       1       TTL
9       1       protocol
10      2       header checksum
12      4       src ip 
16      4       dst ip
20      3       options
23      1       padding
24      1-      data