Which flags exist in the IPv4 Flags field and what are their purposes?

• A. DF (Don't Fragment) and MF (More Fragments); Reserved bit is unused
• B. DF (Don't Fragment) and RF (Retry Fragments); Reserved bit is used
• C. MF (More Fragments) and RF (Route Flap); Reserved bit used
• D. None; there are no flags

Answer: (A)

In IPv4, fragmentation control lives in a 3-bit Flags field that sits with the Fragment Offset in the header. Of these three bits, the reserved bit is not used and should always be zero. The two active flags are Don’t Fragment and More Fragments.

Don’t Fragment tells routers not to fragment the packet. If fragmentation would be required to deliver the packet, it will typically be dropped and an ICMP message may be sent indicating fragmentation is needed but DF is set.

More Fragments indicates whether there are additional fragments of this packet to follow. If More Fragments is 1, more fragments will arrive; if it’s 0, this fragment is the last one.

So the flags that exist and their purposes are Don’t Fragment and More Fragments, with the reserved bit unused. Other options mention flags that aren’t part of IPv4’s Flags field, or claim there are no flags, which isn’t correct.