Although originators may produce fragmented packets, IPv6 routers do not have the option to fragment further. 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. In the first one, The maximum transmission unit is 4000 bytes, and the second it is 2500 bytes. The fragmentation takes place on two levels.

An example of IPv4 multiple fragmentation.