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08: Network Layer - IP header - Differentiated services, Total length, ID, and Flags part 5

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 IP header

Differentiated services :

In 1998 ( 6 years later), TOS saw another change. The first change is that its name has been changed to DS field (Differentiated Services). With this changes, the IP header looks like this :


As shown in the figure, DSCP replaced the first six bits. Its function is similar to  Precedence that was used in TOS. It is, by and large, used to assign a certain priority to a specific packet.

Note : this explanation is enough for you as a CCNA student, although you are not required to know all these details. To know more and deeply about DSCP, see its full lesson.

Total length :

This field occupies 16 bits (2 bytes). It specifies the total length of the IP datagram in bytes. The former is calculated as follows : since the length of the field is 16 bits, we take 2 (which is 2 bytes) and we work out 2 to the power of 16 ( the length of the field) to give 65536 bytes, this number is the length of the IP datagram that must not be exceeded.

Identification ( fragment ID) :

This field occupies 16 bits or 2 bytes. Its function is to identify every fragment uniquely by assigning ID to it. All fragments of a single IP datagram have the same identification number. As you already know, routers split data up into fragments, and each fragment may take different route to the destination, thus arriving out of order. Assigning each fragment a number will make it easier for the destination to reorder the fragments and send each set of fragments to their recipient application.

Flags :

This field is composed of three sub-fields (flags), each one occupies 1 bits in total of 3 bits. As for the first bit, it is reserved for future use. The second flag is DF (Don’t Fragment). A fragment that has this flag set is the last one in the process of transmission. In other words, whenever this flag is to 1 in a given fragment (of an ip datagram), it means that there are no fragments(packets) coming ; it informs the receiver that transmission is finished and that this fragment is the last one. Besides, it determines whether or not a packet is allowed to be fragmented. The last flag is MF (More Fragments). It indicates that there are still fragments coming from the source machine and that the actual fragment is just one of a series of fragments ( of an ip datagram).


This lesson is divided into four parts due to intensive content: first part , second part (which is the current one), third part, fourth part.
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08: Network Layer - IP header - Differentiated services, Total length, ID, and Flags part 5 Reviewed by BOUFTIRA on 9:54:00 AM Rating: 5

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