IPv4 Header – Flags Field (3 bits)

Definition

• The Flags field tells routers and the destination how fragmentation should be handled.
• It contains three control bits: Reserved, DF (Don't Fragment), and MF (More Fragments).
• It works together with the Identification and Fragment Offset fields.

What Problem It Solves

• It lets the sender control whether a packet can be fragmented.
• It also tells the receiver whether the current fragment is the last one or if more fragments are still coming.

What Happens If Not Used

• Routers wouldn't know whether they are allowed to fragment a packet.
• The receiver wouldn't know when it has received the final fragment, making reassembly unreliable.

Easy Wording

It gives instructions about whether a packet can be split and whether more pieces are on the way.

Layman Example

Imagine mailing a large document in several envelopes. One sticker says "Do Not Split" (DF), while another says "More Envelopes Coming" (MF) so the receiver knows to wait before putting everything together.

Technical Example

Request Flow:

• Your computer sends a large HTTPS request.
• The packet reaches a router with a smaller MTU.
• If DF = 1, the router cannot fragment the packet and instead drops it, sending an ICMP error back.
• If DF = 0, the router fragments the packet. Each fragment has MF = 1, except the last fragment, which has MF = 0.
• The destination waits until the last fragment arrives before completing reassembly.

Limitation: The Flags field tells whether fragmentation happens, but not where each fragment belongs in the original packet. Fragment Offset, provides the exact position of each fragment.