IP Fragmentation

The maximum allowed size of an IP datagram differs from one network to the other. This maximum is defined as the maximum transmission unit (MTU). An IP datagram will be fragmented in a router if it is to be forwarded over a network that has an MTU that is smaller than the IP datagram.  All fragments that belong to the same original IP datagram carry the same identifier in the Ident field. This enables these fragments to be reassembled at the receiving host. If one of these fragments gets lost, the receiving host discards all of them. This animation provides an example of IP fragmentation.

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