Figure 1: IPv4 Internet Datagram header Format (Internet Protocol Specification RFC791, Sept. 1981)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version|  IHL    |Type of Service|         Total Length        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Identification          |  Flags  | Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Time to Live  |      Protocol     |      Header Checksum    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                          Source Address                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Destination Address                     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Options                    |    Padding       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version: 4 bits
The Version field indicates the format of the Internet header.
IHL: 4 bits
Internet Header Length is the length of the internet header in 32 bit words,
hence points to the beginning of the data.
Type of Service: 8 bits
Provides an indication of the abstract parameters of the
quality of service desired.
Total Length: 16 bits
Total Length is the length of the datagram, measured in octets.
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.
Fragment Offset: 13 bits
This field indicates where in the datagram this fragment belongs.
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. 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 or may not appear in datagrams.  They must be implemented by all IP modules
(host and gateways). What is optional is their transmission in any particular datagram, not
their implementation. In some environments the security option may be required in all datagrams
Padding:  variable
The Internet header padding is used to ensure that the Internet header ends on a 32-bit
boundary.  The padding is zero.