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Packets used by the IP are called datagrams. For a networks with a smaller MTU, we must divide the datagram to make it possible for it to pass through these networks. This is called fragmentation.

 

A datagram can be fragmented by the source host or any router in the path. The reassembly of the datagram is done only by the destination host.

 

 

In the datagram header, we find three fields related to the fragmentation of the IP datagram. Identification, Flags, Fragmentation Offset)

 

• Identification : helps the destination in reassembling all fragments having the same identification value into one datagram.
• Flags: defines three flags:

1. The leftmost bit is reserved (not used).
2. The second bit (D bit) is called thedo      not fragment bit. If its value is 1, the machine must not fragment the      datagram. If it cannot pass the datagram through any available physical      network, it discards the datagram and sends an ICMP error message to the      source host. If its value is 0, the datagram can be fragmented.
3.  The third bit (M bit) is called themore      fragment bit. If its value is 1, it means there are more fragments      after this one. If its value is 0, it means this is the last or only      fragment.

• Fragmentation Offset: shows the relative position of this fragment with respect to the whole datagram, measured in units of 8 bytes.