IP Maximum Transmission Unit (MTU)
IPv4 routers will fragment automatically to fit within the MTU (unless DF – Don’t Fragment option is set). IPv6 routers will not fragment, and will drop packets that are larger than MTU.
IPv4 requires that every node must be able to forward an IP packet of 68 bytes without any further fragmentation. This is because an IPv4 header can be as large as 60 bytes in length or have a minimum fragment size of 8 bytes. Every IPv4 node that is the final destination of the IPv4 packet must be able to receive an IPv4 packet of a minimum size of 576 bytes, which can be all or fragments of the original packet.
IPv6 requires that every link have a minimum MTU of 1280 bytes, with a recommended MTU of 1500 bytes, compared to 68 bytes in IPv4. It is recommended that IPv6 devices perform Path Discovery MTU to avoid fragmentation.