Calculator Inputs
Example Data Table
This sample uses a 1500-byte packet, 20-byte header, and 600-byte MTU.
| Fragment | Offset Units | Offset Bytes | Data Bytes | Total Length | Start Byte | End Byte | MF Flag |
|---|---|---|---|---|---|---|---|
| 1 | 0 | 0 | 576 | 596 | 0 | 575 | 1 |
| 2 | 72 | 576 | 576 | 596 | 576 | 1151 | 1 |
| 3 | 144 | 1152 | 328 | 348 | 1152 | 1479 | 0 |
Formula Used
Payload Length = Total Packet Length - Header Length
Maximum Fragment Data = floor((MTU - Header Length) / 8) × 8
Fragment Offset = Starting Payload Byte / 8
Fragment Total Length = Header Length + Fragment Data Bytes
More Fragments Flag = 1 for every non-final fragment, and 0 for the final fragment.
IPv4 stores the offset in 8-byte units. Non-final fragments must therefore carry payload sizes aligned to 8 bytes.
How to Use This Calculator
- Enter the original IPv4 packet length.
- Enter the IPv4 header length in bytes.
- Provide the link MTU for the path segment.
- Add a payload override only when needed.
- Choose a fragment number to inspect closely.
- Enable binary output for bit-level review.
- Submit the form to generate the fragment plan.
- Review offsets, flags, byte ranges, and chart output.
- Download the table as CSV or PDF.
Frequently Asked Questions
1. What does the fragment offset represent?
It shows where a fragment’s payload begins within the original payload. IPv4 stores this position in units of eight bytes, not raw bytes.
2. Why is the offset divided by 8?
The IPv4 header allocates 13 bits for the fragment offset. Using 8-byte units expands the addressable payload range while keeping the header compact.
3. Why must non-final fragments align to 8 bytes?
Because the next fragment’s offset must still land on an exact 8-byte boundary. This keeps reassembly positions correct for every fragment except the last one.
4. Does the last fragment need 8-byte alignment?
No. The last fragment can carry the remaining payload bytes, even when that amount is not divisible by eight.
5. What happens when the DF flag is enabled?
DF means “Don’t Fragment.” If the packet exceeds the MTU, the router drops it and usually returns an ICMP message instead of fragmenting it.
6. Does header length change the fragment offset?
Header length does not directly change the offset value. It reduces available payload space per fragment, which indirectly affects how many fragments are created.
7. What does the MF flag mean?
MF means “More Fragments.” A value of 1 means another fragment follows. A value of 0 marks the final fragment in the sequence.
8. Why would no fragmentation appear in the result?
If the packet total length is already less than or equal to the MTU, the packet fits the link as-is, so fragmentation is unnecessary.