Calculator Inputs
Example Data Table
| Fragment | Start Byte | End Byte | Data Bytes | Offset Units | Total Length | MF Flag |
|---|---|---|---|---|---|---|
| 1 | 0 | 1479 | 1480 | 0 | 1500 | 1 |
| 2 | 1480 | 2959 | 1480 | 185 | 1500 | 1 |
| 3 | 2960 | 3999 | 1040 | 370 | 1060 | 0 |
Example assumes 4000 payload bytes, 1500-byte MTU, and 20-byte IPv4 header.
Formula Used
1) Maximum data per fragment
Max Data = floor((MTU − IP Header Length) / 8) × 8
2) Fragment offset field
Fragment Offset = Starting Byte Position / 8
3) Total length of a fragment
Fragment Total Length = IP Header Length + Fragment Data Bytes
4) More Fragments flag
MF = 1 for every fragment except the last one. The final fragment uses MF = 0.
The offset field is stored in 8-byte units, not raw bytes. That is why fragment boundaries must line up on 8-byte multiples.
How to Use This Calculator
- Enter the total IPv4 payload size in bytes.
- Enter the MTU for the link or path.
- Enter the IPv4 header length. Use 20 for a standard header.
- Choose which fragment you want to inspect.
- Optionally enter a manual starting byte to test offset alignment.
- Press Calculate Fragment Offset.
- Review the result summary, graph, and full fragment table.
- Use the CSV or PDF buttons to export the calculated breakdown.
Frequently Asked Questions
1. What does fragment offset mean in IPv4?
It shows where a fragment’s payload begins inside the original IP payload. The field uses 8-byte units, so the stored value is the starting byte divided by 8.
2. Why is the offset not stored in bytes?
IPv4 uses a compact header format. Storing the offset in 8-byte units saves header bits while still identifying fragment positions accurately for reassembly.
3. Why must most fragments use data sizes divisible by 8?
Every following fragment must begin at a valid offset boundary. Using payload sizes divisible by 8 keeps the next fragment’s starting byte aligned with the offset field format.
4. Does the final fragment also need an 8-byte multiple?
No. The last fragment may carry any remaining number of bytes. Only the earlier fragments must keep the next starting position aligned for later offsets.
5. Should the payload value include TCP or UDP headers?
Yes. The IP payload includes everything after the IPv4 header, including transport-layer headers and application data carried inside the packet.
6. What is the More Fragments flag?
MF indicates whether more fragments follow. A value of 1 means another fragment exists later. A value of 0 identifies the last fragment.
7. Why can fragmentation reduce efficiency?
Each fragment carries its own IPv4 header. More fragments mean more overhead, so the percentage of useful payload versus transmitted bytes becomes smaller.
8. When is this calculator useful?
It helps with MTU planning, packet analysis, lab exercises, protocol troubleshooting, security reviews, and any networking task that requires understanding IPv4 fragmentation behavior.