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If an IP packet exceeds the MTU of any interfaces it has to traverse (the path MTU), then any intermediate router may fragment the packet into multiple datagrams. Each datagram must be no greater than the path MTU. The sender can optionally set the don't fragment (DF) bit in the IP header to prevent intermediate routers from fragmenting the packet.

IPv6 differs from IPv4 when it comes to fragmentation. IPv4 packets can be fragmented by any router along the path unless the DF bit is set. IPv6 can be fragmented only by the sender. If an IPv6 packet will exceed an intermediate router's interface MTU, the router will respond to the sender with an ICMPv6 “packet too big” message and discard the packet.