a ip header:

        0                                             15 16                                            31
       --------------------------------------------------------------------------------------------------
       |  4-bit  | 4-bit header | 8-bit type of service |          16-bit total length                  |
       | version |   length     |         (TOS)         |              (in bytes)                       |
       --------------------------------------------------------------------------------------------------
       |            16-bit identification               | 3-bit |      13-bit fragment offset           |
       |                                                | flags |                                       |
       --------------------------------------------------------------------------------------------------
       |  8-bit time to live    |   8-bit protocol      |       16-bit header checksum                  |
       |        (TTL)           |                       |                                               |
       --------------------------------------------------------------------------------------------------
       |                                   32-bit source IP address                                     |
       --------------------------------------------------------------------------------------------------
       |                                 32-bit destination IP address                                  |
       --------------------------------------------------------------------------------------------------

       protocol:
           1 - ICMP
           2 - IGMP
           6 - TCP
           9 - IGRP
          17 - UDP
          47 - GRE
          50 - ESP
          51 - AH
          88 - EIGRP
          89 - OSPF
         115 - L2TP