Calculator Input
The page uses a single column flow. Only the calculator fields use a responsive grid.
Example Data Table
| Version | IHL | DSCP | ECN | Payload | ID | Flags | Offset | TTL | Protocol | Source | Destination | Options Hex | Header Bytes | Total Bytes |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 4 | 6 | 46 | 0 | 1200 | 54321 | DF=1, MF=0 | 0 | 64 | 6 (TCP) | 192.168.1.10 | 203.0.113.25 | 01010100 | 24 | 1224 |
| 4 | 5 | 0 | 0 | 512 | 12345 | DF=1, MF=0 | 0 | 64 | 17 (UDP) | 10.0.0.5 | 8.8.8.8 | None | 20 | 532 |
Use these rows to verify field placement, byte growth, and overhead changes.
Formula Used
Header Length
Header Length = IHL × 4 bytes
Total Length
Total Length = Header Length + Payload Length
Options Length
Options Length = (IHL − 5) × 4 bytes
TOS Byte
TOS = (DSCP × 4) + ECN
Fragment Start Position
Fragment Start Byte = Fragment Offset × 8
Header Checksum
Checksum = One’s complement of the one’s complement sum of all 16-bit header words, with the checksum field set to zero during calculation.
Header Overhead
Overhead % = (Header Length ÷ Total Length) × 100
How to Use This Calculator
- Enter the IHL value in 32-bit words.
- Add DSCP and ECN values for service marking.
- Provide payload length, identification, and TTL.
- Choose the transport or routing protocol number.
- Enter source and destination IPv4 addresses.
- Add options hex only when IHL is above 5.
- Set DF, MF, and fragment offset when needed.
- Press calculate to see decoded fields, checksum, graph, and export options.
FAQs
1. What does IHL mean?
IHL means Internet Header Length. It tells how many 32-bit words exist in the IPv4 header. Multiply IHL by four to get bytes.
2. Why is the minimum IPv4 header 20 bytes?
A standard IPv4 header has five 32-bit words. Five words multiplied by four bytes equals 20 bytes. That size excludes any optional fields.
3. What do DF and MF flags control?
DF blocks fragmentation. MF signals that more fragments follow. Together with the fragment offset, they describe how a packet is split and reassembled.
4. What is the fragment offset measured in?
The fragment offset uses 8-byte units. A value of 100 means the fragment payload begins 800 bytes after the original data start.
5. What does the header checksum cover?
The IPv4 header checksum protects only the header. It does not validate payload contents. Routers recalculate it whenever header values change.
6. Why would I use options hex?
Options hex lets you model optional IPv4 header bytes. It is useful for labs, packet analysis, legacy testing, and header size verification.
7. Does this calculator work for IPv6 headers?
No. This page models IPv4 headers only. IPv6 uses a different base header format and does not use the same checksum field.
8. What does the chart show?
The chart compares header bytes, options bytes, payload bytes, and total bytes. It helps you quickly see overhead and structural balance.