Internet Protocol version four - The version of internet protocol used widely on the Internet. It assigns a 32 bit address to every machine connected to the Internet and allows packets to be routed from their source machine to their destination machine over the network of networks known as the Internet.

IPv4 has been fully specified in RFC 791 in 1981. It was the first widely used protocol version. It defines a header consisting of a version number (4), a destination address, a source address and a variety of fields used to help rout the packet's data across the internet.

Here is the IPv4 protocol header bits represented in table form:

bit	0 - 3				4 - 7				8 - 15								16 - 18			19 - 31
0	Version				Header length				Type of Service (newer: DiffServ or ECN)								Total Length
32	Identification																Flags			Fragment Offset
64	Time to Live								Protocol								Header Checksum
96	Source Address
128	Destination Address
160	Options
192	Data

Version

The protocol version number for the header format. Currently it can be 4 for IPv4 or 6 for IPv6.


Header Length



Type of Service (ToS)
see: Type of Service


Total Length
Total Length of packet


Identification


Flags


Fragment Offset


Time to Live (TTL)
Was once specified in seconds but now means number of router to router hops permitted to be taken by the packet before it is discarded.

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