A 3-bit field of the IP packet header
The first bit is unused. The second is the do not fragment
) bit. When that is set to one, a router cannot fragment the packet. If the packet can't be forwarded without fragmenting
, the router drops the packet and sends an error message to the source. This allows for MTU
testing in an internetwork. The third (low order bit is the more fragments
) bit, which specifies if the fragment contains data from the middle or the end of the original datagram. When a router fragments a packet, it sets the MF bit to one in all but the last fragment. The receiver expects fragments until it receives a fragment with MF=0.