Hello Narad
The header length field of the IPv4 packet is always 4 bits. That means it can have a value between 0 and 15. But remember, it signifies the number of 32-bit blocks that the header contains. When there are no options fields, the actual lowest value that you can have for this field is 5 in decimal or 0101 in binary. This is because that’s the smallest IPv4 header you can have.
The Identification field is used to aid in the reassembly of fragmented IP packets. But it’s not quite as simple as that. This value must be the same for:
- a given source address
- a given destination address
- a given protocol
- for the duration of the maximum datagram lifetime (MDL) which is typically 120 seconds
The fragments of any fragmented IP packet will have the same source and destination addresses, and of course the same protocol, and must also all be sent within the limit of the MDL. If those criteria are matched, then the ID will indeed be the same for all fragments of the IP packet. What changes is the offset value.
More information bout this can be found at the following links:
I hope this has been helpful!
Laz