Q & A on Fragmentation Q: Why is fragmentation needed on an internet but not on a typical Wide Area Network? A: Fragmentation is needed whenever a datagram of size K is forwarded across a network with MTU less than K. If one is sending across a single WAN, the datagram size can be chosen by the source to be less than the MTU. If one is sending across an Internet, the sender does not know the MTU of intermediate networks, and so cannot guarantee that datagrams will be smaller than the smallest MTU.