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fragmentation is done by network layer  and  generally  by  a router

 and it is required because if there is network where MTU is less than  paket size

then  there fragmentation is required.

 

fragmentation is nothing but  dividing paket into fragments so new pakets could could fit into a frame

whose MTU is less than original packet

 

it  is never done  on  source computer,  it  is  always  done by routers  and  it  is done  at layer 3  -- network layer

 

on   tranport  layer (source computer )  segmentation is done  such that it  gets  easily fit into  FRAME of data link layer

so to avoid fragmentation on this DLL(dat link layer).
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fragmentation is done by network layer as per according to size of MTU(MAXIMUM transfer unit).

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Fragmentation : When a packet of large size (Larger than the underlying layer MTU) splits into smaller packets such that lower layer MTU get satisfied by each packet. These smaller packets are called Fragments and this process is called Fragmentation.

Fragmentation is done at Network layer but which device’s network layer is the question, Host or Router?

Host have all 7 layers running in it’s OSI model stack. Among all the layers, TCP layer is the most reliable and the  smartest one. It notes the bottleneck bandwidth of underlying layers and choose the minimum one and decide the segment size accordingly. Suppose, the bottleneck bandwidth is at network layer. it will make segment of the size such that each segment will fit into datagram, Or if the the bottleneck bandwidth  is at data link layer it will make segment of the size such that each segment will fit into frame. This helps preventing task of doing Segmentation and Fragmentation both.

But router is a layer-3 device, which mean the highest layer it has is network layer. So, if one side router is receiving a frame of 1500 Bytes and on the other side MTU is >=1500 B then no need for fragmentation. But if the other side data link layer has MTU = 500 Bytes, then fragmentation will be done by router before sending datagram to that link.

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