information:classification, information:highlighting
IPv4 and IPv6 differences

Under IPv4, a router that receives a network packet larger than the next hop's MTU has two options: drop the packet if the Don't Fragment (DF) flag bit is set in the packet's header and send an Internet Control Message Protocol (ICMP) message which indicates the condition Fragmentation Needed (Type 3, Code 4), or fragment the packet and send it over the link with a smaller MTU. Although originators may produce fragmented packets, IPv6 routers do not have the option to fragment further. Instead, network equipment is required to deliver any IPv6 packets or packet fragments smaller than or equal to 1280 bytes and IPv6 hosts are required to determine the optimal MTU through Path MTU Discovery before sending packets.