IP Header

Defined at RFC 791
A summary of the contents of the internet header follows:



Version: 4 bits
The Version field indicates the format of the internet header.
4 - ipv4
6 - ipv6

IHL: 4 bits
Internet Header Length is the length of the internet header in 32
bit words, and thus points to the beginning of the data.
5 - minimum value without option

Type of Service: 8 bits
Bits 0-2: Precedence.
Bit 3: 0 = Normal Delay, 1 = Low Delay.
Bits 4: 0 = Normal Throughput, 1 = High Throughput.
Bits 5: 0 = Normal Relibility, 1 = High Relibility.
Bit 6-7: Reserved for Future Use.

0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| | | | | | |
| PRECEDENCE | D | T | R | 0 | 0 |
| | | | | | |
+-----+-----+-----+-----+-----+-----+-----+-----+

Precedence
111 - Network Control
110 - Internetwork Control
101 - CRITIC/ECP
100 - Flash Override
011 - Flash
010 - Immediate
001 - Priority
000 - Routine

Total Length: 16 bits
Total Length is the length of the datagram, measured in octets, including internet header and data.

Identification: 16 bits

An identifying value assigned by the sender to aid in assembling the
fragments of a datagram.

Flags: 3 bits
Various Control Flags.
Bit 0: reserved, must be zero
Bit 1: (DF) 0 = May Fragment, 1 = Don't Fragment.
Bit 2: (MF) 0 = Last Fragment, 1 = More Fragments.

0 1 2
+---+---+---+
| | D | M |
| 0 | F | F |
+---+---+---+

Fragment Offset: 13 bits

This field indicates where in the datagram this fragment belongs.
The fragment offset is measured in units of 8 octets (64 bits). The
first fragment has offset zero.

Time to Live: 8 bits

This field indicates the maximum time the datagram is allowed to
remain in the internet system.

Protocol: 8 bits
This field indicates the next level protocol used in the data
portion of the internet datagram. The values for various protocols
are specified in "Assigned Numbers" RFC 1700
1 - ICMP
2 - IGMP
6 - tcp
17 - udp
47 - GRE
88 - IGRP


Header Checksum: 16 bits

A checksum on the header only. Since some header fields change
(e.g., time to live), this is recomputed and verified at each point
that the internet header is processed.

Source Address: 32 bits
The source address.

Destination Address: 32 bits
The destination address.

Options: variable
The options may appear or not in datagrams.