IPv4 and IPv6 fragmentation
When an IPv4 packet is larger than the network's MTU(default to 1500), and the DF bit within the IP header is clear the packet will be fragmented into smaller pieces so it can be sent to the other end. The maximum size of each piece is the MTU minus the IP header size (20 bytes minimum; 60 bytes maximum).
Fragmentation has a negative impact on router's performance and it should be avoided when possible.
IPv6 Fragmentation-
As opposed to IPv4, fragmentation in IPv6 is performed by the IPv6 enabled nodes not by the routers along the path. If an intermediate node such as a router receives an IPv6 packet that needs to be fragmented, it will discard the packet and send an ICMPv6 Packet Too Big error message back to the source as routers will not attempt to perform fragmentation unless they are the source of the IPv6 packet.
One of the innovations introduced by IPv6 is the elimination of hop-by-hop packet fragmentation. With the new protocol, fragmentation is managed at the ends by means of a special extension header.
More specifically, there are two main differences: Difference one is the fields for handling fragmentation are not in the basic IPv6 header but are put into an extension header if fragmentation is required. This makes IPv6 fragmentation lean because this fragmentation extension header is only inserted in the packet, if fragmentation needs to be done. Difference two is that IPv6 routers do not fragment anymore. Fragmentation has to be done by the source host. He will evaluate the packet size by using Path MTU discovery.
http://searchnetworking.techtarget.com/answer/What-is-the-difference-between-packet-fragmentation-in-IPv4-and-IPv6
https://blog.apnic.net/2016/01/28/evaluating-ipv4-and-ipv6-packet-frangmentation/
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