Length of DHCP fields in octets, and their placement in packet.
Ref: http://4.bp.blogspot.com/-IyYoFjAC4l8/UXuo16a3sII/AAAAAAAAAXQ/b6BojbYXoXg/s1600/DHCPTitle.JPG
0  OP - 1
1  HTYPE - 1
2  HLEN - 1
3  HOPS - 1
4  XID - 4
5  SECS - 2
6  FLAGS - 2
7  CIADDR - 4
8  YIADDR - 4
9  SIADDR - 4
10 GIADDR - 4
11 CHADDR - 6
12 MAGIC COOKIE - 10
13 PADDING - 192 octets of 0's
14 MAGIC COOKIE - 4
15 OPTIONS - variable length