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Understanding TCP/IP by Libor Dostálek, Alena Kabelová

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5.3 Fragmentation

IP datagrams are wrapped into link frames. The link protocols only enable data transmission within their frames up to a certain maximum limit. The maximum data size that can be inserted into one link frame is called Maximum Transfer Unit (MTU).

Link Protocol

MTU

Ethernet II

1500

Ethernet 802.3 SNAP

1492

Frame Relay

1600

FDDI

4478

It is obvious from looking at the previous table that most link protocols have MTU in ones of kilobytes. On lines connecting remote locations, we also sometimes encounter an MTU smaller than 1 KB. The total field length of an IP datagram, however, is 16 bits long; so theoretically, it is possible to create an IP datagram up to 64 KB long.

But what happens when an IP datagram on route from ...

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