June 21, 2018

Understanding the IPv4 header

IPv4 is the fourth version of Internet Protocols and perhaps the first version to be deployed so widely.

Along with IPv6, both protocols are at the core of standards based networking techniques.

IPv4 is a connectionless protocol for use on Packet Switched Link Layer Networks. IPv4 functions around a ‘best effort delivery approach’ which means that it neither guarantees delivery nor does it avoid duplicate delivery or ensure adequate sequencing. These aspects and others closely related are addressed by an Upper Layer Transport Protocol, like the Transmission Control Protocol.

The IPv4 packet header consists of 20 bytes of data and a total of 14 fields, with the 14^th one being optional.

The Version

The first header field in an IP packet is a four-bit version field. This typically indicates and dictates the format. The version also identifies the IP version to which the packet belongs. It could be either set to 0110 to indicate version 6 or 0100 to indicate IPv4.

Internet Header Length

The second field is the internet header length, indicating the numbers of 32-bit words. Since the IPv4 header may contain a different number of options, this field typically specifies the size of the header and the minimum value for this field is 5.

Type Of Service

Also referred to as Differentiated Services Code Point, this field carries information to provide quality of services features. There are new technologies constantly emerging that require real time data streaming and therefore make constant use of this field. The perfect example is VoIP that is used for data voice exchange.

Explicit Congestion Notification

This allows notification of congestion with dropping packets. This is an optional feature that can only be used when both points support it

Total Length

This 16-bit field defines the overall datagram size in bytes.

Identification

As the name implies, this is an identification field that identifies fragments of original IP datagram.

Flags

A three-bit field that is used to control the fragments. They are in a high to low order.

Don’t Fragment

Sets the don’t fragment bit in packets being sent. When a data flag is set to don’t fragment, the intermediate devices are not allowed to fragment it.

More Fragments

This is set to indicate that the current datagram is a set of a larger datagram.

Fragment Offset

The fragment offset field, measured in eight-byte long fields is 13 bits long and indicates the offset of a specific fragment relative to the beginning unfragmented IP datagram.

Time To Live

This field shows the maximum time the datagram is allowed to remain in the field.

Protocol

This field indicates the protocol used in the data portion of the IP datagram.

Header Checksum

This checksum field is used to check errors present in the header.

There you have it—a quick breakdown of the IPv4 header. At IPv4 Mall, we’re a leading online platform that allows you to sell and buy IPv4 address blocks. Contact us for more information.