When an IP datagram is too large for the maximum transmission unit (MTU) of the underlying data link layer technology used for the next leg of its journey, it must be fragmented before it can be sent across the network. The higher-layer message to be transmitted is not sent in a single IP datagram but rather broken down into pieces called fragments that are sent separately. In some cases, the fragments themselves may need to be fragmented further.

Fragmentation is necessary to implement a network-layer internet that is independent of lower layer details, but introduces significant complexity to IP. Remember that IP is an unreliable, connectionless protocol. IP datagrams can take any of several routes on their way from the source to the destination, and some may not even make it to the destination at all. When we fragment a message we make a single datagram into many, which introduces several new issues to be concerned with:

• Sequencing and Placement: The fragments will typically be sent in sequential order from the beginning of the message to the end, but they won't necessarily show up in the order in which they were sent. The receiving device must be able to determine the sequence of the fragments to reassemble them in the correct order. In fact, some implementations send the last fragment first, so the receiving device will immediately know the full size of the original complete datagram. This makes keeping track of the order of segments even more essential.
  
  
• Separation of Fragmented Messages: A source device may need to send more than one fragmented message at a time; or, it may send multiple datagrams that are fragmented en route. This means the destination may be receiving multiple sets of fragments that must be put back together. Imagine a box into which the pieces from two, three or more jigsaw puzzles have been mixed and you understand this issue.
  
  
• Completion: The destination device has to be able to tell when it has received all of the fragments so it knows when to start reassembly (or when to give up if it didn't get all the pieces).

To address these concerns and allow the proper reassembly of the fragmented message, IP includes several fields in the IP format header that convey information from the source to the destination about the fragments. Some of these contain a common value for all the fragments of the message, while others are different for each fragment.