What is DF bit in IP header?

DF means ‘Don’t Fragment‘.There are three-bit field follows and is used to control or identify fragments.

bit 0: Reserved; must be zero.
bit 1: Don’t Fragment (DF)
bit 2: More Fragments (MF)

If the DF flag is set & fragmentation is required to route the packet, then the packet is dropped. This can be used when sending packets to a host that does not have sufficient resources to handle fragmentation.

For unfragmented packets, the MF flag is cleared. For fragmented packets, all fragments except the last have the MF flag set. The last fragment has a non-zero Fragment Offset field, differentiating it from an unfragmented packet.