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What is an IPv4 datagram?

  • Posted at 2022-06-01 19:17:03
  • By Prefixx Team

What is an IPv4 datagram?

Internet Protocol (IP) is a communication method in the network layer used to exchange data. The primary objective of the IP is to route the information or packets produced at the host computer and send it to the destination router.

The transport layer is responsible for processing the data at the receiving end. The transport layer breaks the data into small packets of information. After that, the packets with the data are stored and sent through the IP routers to the destination. When the packets are delivered and finally reassembled into the original stream at the destination.

Internet networks are built around a mesh-like structure where everything device is connected securely. The hardware used in making the network varied, affecting the network's bandwidth. 

Therefore, the IP has to break the packets into smaller parts to provide a seamless experience to the receiving users. The process involved in breaking the information into small pieces is called fragmentation. 

Data security over the network is taken seriously to avoid hacking and data loss during transit. The process of sending and receiving data through IP works at lightning speed. It ensures the sender's packets are original packets and not the manipulated ones. Also, multiple security layers are added.

IPv4 Datagram

IPv4 was first introduced to the world in the year 1981. Since then, the technology is still widely used in internet services. The IPv4 uses 32-bit address space.

A datagram is a name given to the IPv4 packets produced when the information is sent from the host device to the receiving device. A datagram is made of two sections header and data parts. The header consists of the 20-byte fixed obligatory part with the extra fields. 

Therefore, the minimum size of the IPv4 header always remains at 20-bytes. Moreover, the options and padding occupy the optional non-used part in the header. The remaining field is used to grow the capacity of the header up to 40-bytes in size. So when both elements are combined, the header will reach the size of 60 bytes.

A datagram holds the carrying capacity of up to 65,535 bytes of data () where at least 20 bytes should be in the header section. Thus, when calculated, you will get the maximum carrying capacity of the data up to 65,515 bytes.

Now, if we talk about the fields section in the header, it contains 14 different fields. In which the 13 are used as mandatory fields. And remaining one is called an optional field known as options.

The following information describes the individual header fields.


 The current IP version field is set to 4 for IPv4. The latest version is upgraded to meet modern demand. 

Internet Header Length

Internet Header Length is the total number of 32-bit words consisting of the header field. The minimum number of fields required is five because 20 bytes are mandatory for performing the essential operation.

Differentiated Services Code Point (DSCP)

DSCP defines the type of service used to differentiate the services, such as voice-over IP.

Explicit Congestion Notification (ECN)

ECN offers seamless functionality to the network while carrying the packets. It runs the process without dropping the packets or utilizing the excess bandwidth.

Total Length

It counts the datagram's total size, including the header and data parts.


The feature identifies the group of fragments when the received datagram at the receiving end is fragmented.


Different Flags are used to control the fragmentation during the data transition from one network to another. It indicates the datagram fragmentation. For example, when the don't fragment flag is applied, it means the destination device cannot reassemble the received fragmented packet because of instruction. 

Fragment Offset

It indicates the offset of the fragmented packet in the network.

Time to Live (TTL)

While passing through the routers, the packets are decremented in the TTL field. It shows the number of times the packets are hopped during the transition. In case the number reaches zero, the packets will be discarded. By using this process, looping of the packets is avoided.


The term uses to describe the protocol for the data part.

Header Checksum

Header checksum is the quality checking procedure applied to verify the packets received at the router. It consists of only a header field. Every router has this header checksum protocol to ascertain the value of the packets before sending them to the next junction. If the error is detected in the packets, they are immediately discarded. 

Source Address

It describes the source host IPv4 address.


The network communication protocol is used in all our multimedia devices that send and receive information through the internet facility. Understanding the datagram structure will help you understand the role of the routers, packets, header, and fields. Keep learning to know more.

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