Internet Checksum

Calculate the 16-bit one's-complement Internet checksum used by IP, TCP, UDP, and related protocols over UTF-8 bytes.

Description

Calculate Internet Checksum over UTF-8 text and display the result as uppercase hexadecimal.

Internet Checksum is intended for interoperability and accidental-error detection. The Internet checksum groups bytes into big-endian 16-bit words, adds them with end-around carry, and complements the final sum.^[1]

When to use Internet Checksum

Use this checksum only when a file format, device, protocol, or published test vector specifies the same named profile and parameters. Profiles with the same width can still produce unrelated results.

How Internet Checksum works

The Internet checksum groups bytes into big-endian 16-bit words, adds them with end-around carry, and complements the final sum.^[1]

How the Internet function processes input

Encode the entered text as UTF-8 bytes.
Process the bytes with the selected algorithm and its exact parameters.
Render the result in the canonical textual form shown by the tool.

Limitations and assumptions

Checksums are not collision-resistant and must not be used as digital signatures or security proofs.
Text is encoded as UTF-8; a checksum over a file's raw bytes can differ.
Every CRC parameter—including reflection and initialization—must match the other implementation.

Alternative or Complementary approaches

Use a cryptographic hash such as SHA-256 when malicious modification is in scope. Use the custom CRC calculator when a documented profile is not available as a preset.

References

RFC 1071: Computing the Internet Checksum — Internet Engineering Task Force

Similar or alternative tools

CRC-32 Checksum
Use the common ISO-HDLC CRC-32 profile.
SHA-256 Hash Generator
Use a cryptographic digest for adversarial integrity.