IPv4 Header – Fragment Offset Field (13 bits)

Definition

• The Fragment Offset field tells the destination where a fragment's data belongs within the original IPv4 packet.
• It is used only when a packet has been fragmented and works with the Identification and Flags fields to reassemble the packet correctly.

What Problem It Solves

• Fragments may arrive out of order because they can take different network paths.
• The Fragment Offset tells the receiver the correct position of each fragment, so the original packet can be rebuilt accurately.

What Happens If Not Used

• The receiver would know the fragments belong together (from the Identification field) but not where each piece should be placed.
• The reassembled packet could become corrupted or incomplete.

Easy Wording

It tells the receiver where each piece of a split packet fits in the original message.

Layman Example

Think of a book delivered in separate packages. Each package has a note saying "Starts at page 101." Even if the packages arrive in a different order, you know exactly where each section belongs.

Technical Example

Request Flow:

*A large file upload creates an IPv4 packet bigger than the network's MTU.

• The router fragments the packet.
• Each fragment gets an Identification value and a Fragment Offset (stored in 8-byte units). *The fragments may arrive at the web server in a different order
• The server uses the Fragment Offset to place each fragment in its correct position before reassembling the original packet.

Limitation: Fragment Offset only helps with fragmented packets. It doesn't prevent packets from looping endlessly through the network. Time To Live (TTL), solves that problem.