IPv4 Packet Structure - Internet Protocol version 4 (IPv4)

What is the use of IPv4 Packet Structure?

As internet Protocol is a layer-3 protocol (OSI), it will take data Segments from layer-4 (Transport) and divide it into packets. IP packet will encapsulate data unit that are received from above layer and it adds to its own header information.


The encapsulated data can be referred as IP Payload and IP header will consist of all the required information for delivering the packet at the other end.


IP header consists of much relevant information including Version Number, which, in this context, is 4. Other details are as follows:

  • Version: Internet Protocol Version no. used (e.g. IPv4).
  • IHL: length of Internet Header and entire IP header Length.
  • DSCP: Differentiated Services Code Point; this is Type of Service.
  • ECN: Explicit Congestion Notification; It will carry the information about the congestion seen in the route.
  • Total Length: Entire IP Packet length (including IP header and IP Payload).
  • Identification: If IP packet is fragmented during the transmission, all the fragments include same identification number for identifying the original IP packet to which they belong.
  • Flags: As needed by the network resources, if IP Packet is too large to handle, these ‘flags’ will tell us if they can be fragmented or not. In this 3-bit flag, the MSB will always be set to ‘0’.
  • Fragment Offset: This offset will tell the exact position of the fragment in the original IP Packet.
  • Time to Live: To avoid looping in the network, every packet will be sent with some TTL value set, which tells the network how many routers (hops) this packet can cross. At each hop, its value will be decremented by one and when the value reaches zero, the packet is discarded.
  • Protocol: Tells the Network layer at the destination host, to which Protocol this packet belongs to, i.e. the next level Protocol. For example protocol number of ICMP is 1, TCP is 6 and UDP is 17.
  • Header Checksum: This field will be used for keeping checksum value of entire header which is then used for checking if the packet is received error-free.
  • Source Address: 32-bit address of the Sender (or source) of the packet.
  • Destination Address: 32-bit address of the Receiver (or destination) of the packet.
  • Options: This is optional field used when the value of IHL is greater than 5. These options can consist of values for options such as Security, Record Route, Time Stamp, etc.

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Internet Protocol version 4 (IPv4) Topics