The remaining fields are used to identify and validate the packet, or to reorder a fragmented packet.

The fields used to identify and validate the packet include:

A router may have to fragment a packet when forwarding it from one medium to another medium that has a smaller MTU. When this happens, fragmentation occurs and the IPv4 packet uses the following fields to keep track of the fragments:

Note: The Options and Padding fields are rarely used and beyond the scope of this chapter.