IPv4 Header – Identification Field (16 bits)

Definition

• The Identification field gives every IPv4 packet a unique ID number so that if the packet is fragmented, all of its pieces can be recognized as belonging to the same original packet.
• It works together with the Flags and Fragment Offset fields during fragmentation and reassembly.

What Problem It Solves

• Sometimes a packet is too large for a network link's MTU (Maximum Transmission Unit).
• The router splits the packet into smaller fragments.
• The Identification field lets the destination know which fragments should be joined back together.

What Happens If Not Used

• If multiple fragmented packets arrived at the same time, the receiver wouldn't know which fragments belonged together.
• This could result in corrupted or incomplete data.

Easy Wording

It gives a packet a "group number" so its pieces can be put back together correctly.

Layman Example

Imagine a 500-piece puzzle shipped in three boxes. Each box has Puzzle #125 printed on it, so you know all three boxes belong to the same puzzle.

Technical Example

Request Flow:

• Your browser uploads a large file.
• The IPv4 packet is larger than the next network's MTU.
• The router fragments the packet and assigns Identification = 700 to every fragment.
• The destination receives all fragments with ID = 700 and groups them together for reassembly.

Limitation: The Identification field only says which fragments belong together. It doesn't indicate whether fragmentation is allowed or whether more fragments are coming. Flags, provides those instructions.