What is IP Fragmentation?

--> When a device sends IP Packet to another device on the network, it should not be bigger than MTU value.

--> MTU stands for Maximum Transmission Unit.

--> The Maximum size of the MTU is 1500 bytes.

--> If the IP Packet of more than 1500 bytes then we need to divide this packet into smaller packets that are equal or less than MTU before sending to the other device.

--> This process is called as Fragmentation.

--> In order to identify each and every fragmented packet on the other device ( Receiving host ), Sending device adds fragmented ID for each packet for reassembly.

--> Fragmentation ID is actually a copy of IP identification Field that is present in IP Header.

--> Packets won't be reassembled until all the fragmented packets reach on the destination.

--> Each and every fragmented packet contains three items with it,

i) Offset -- specifies the location or place of the fragmented packet

ii) Length -- Size of the data

iii) More Fragments -- Indicates whether the next packet is fragmented packet or not.

--> If More Fragments Flag is equal to One specifies fragmented packets are coming next.

--> If More Fragments Flag is equal to zero specifies fragmented packets are finished.

Ex: If you want to send 4000 bytes data to other devices with MTU of 1500.

--> IP Header contains 20 bytes which will be added to DATA before sending to another device.

Fragmented Packet1 : IP header(20) | Data ( 1500) | ID = 88, Len = 1500, off = 0, MF = 1


Fragmented Packet2 : IP header(20) | Data ( 1500) | ID = 88, Len = 1500, off = 20, MF = 1


Fragmented Packet3 : IP header(20) | Data ( 1000) | ID = 88, Len = 1000, off = 0, MF = 0

ID = IP Identification Number

Len = Data Length

off = offset

MF = More Fragment