Please refer to RFC791 for the complete Internet Protocol(IP) Specification.

 

IPv4 Header

0	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
Version				IHL(Header Length)				TOS(Type of Service)								Total Length
Identification																IP Flags			Fragment Offset
TTL(Time To Live)								Protocol								Header Checksum
Source IP Address
Destination IP Address
IP Option(variable length, optional, not common)

Version

Version of IP Protocol. 4 and 6 are valid. This diagram represents version 4 structure only.

 

IHL(Header Length)

Number of 32-bit words in TCP header, mininum value of 5. Multiply by 4 to get byte count. range 5 to 15.

 

TOS

8	9	10	11	12	13	14	15
Precedence			D	T	R	M	U

D: Delay; T: Throughput; R: Reliability; M: Monetary; U: Unused.

 

Total Length

Total length of IP datagram, or IP fragment if fragmented. Measured in Bytes.

 

IP Flags

48	49	50
X	D	M

X: 0x80 reserved(evil bit)

D: 0x40 Do Not Fragment(0 permit fragment, 1 deny fragment)

M: 0x20 More Fragments follow(0 present no fragment or last fragments, 1 preset more fragments)

 

Fragment Offset

 Fragment offset from start of IP datagram. Mesured in 8 byte(2 word, 64 bits) increments. If IP datagram is fragmented, fragment size(Totol Length) must be a multiple of 8 bytes.

 

Protocol

IP Protocol ID. Including(but not limited to):

1.ICMP    2.IGMP    6.TCP    9.IGRP    17.UDP    47.GRE    50.ESP    51.AH    57.SKIP    88.EIGRP    89.OSPF    115.L2TP

 

Header Checksum

Checksum of entire IP header.