A 3-bit field of the IP packet header.

The first bit is unused. The second is the do not fragment (DF) 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 (MF) 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.