Fragmentation and Reassembly
Need to accomodate different physical network maximumn packet sizes
e.g. Ethernet 1.5 kB and FDDI 4.5 kB.
Every network type has a Maximum Transmission Unit (MTU) which is the largest IP datagram that it can carry in a frame.
If MTU decreases over part of a network, it may be necessary to fragment the packet.
Example: 1420 Byte datagram (20 Byte IP header plus 1400 Bytes of data) from Host A to Host B