Internet Protocol Checksum Calculator

What Is an Internet Protocol Checksum?

The Internet checksum is a 16-bit error-detection value used by several Internet protocols. It is based on one’s-complement addition rather than a polynomial CRC.

RFC 1071 defines the Internet checksum by grouping adjacent octets into 16-bit integers, adding those words using one’s-complement arithmetic, and taking the one’s complement of the resulting 16-bit sum when generating the checksum. :contentReference[oaicite:1]{index=1}

The same checksum family is used by IPv4 header checksums and in TCP and UDP checksum calculations, although TCP and UDP also include protocol-specific pseudo-header data in their complete checksum coverage.

How to Use the Internet Checksum Calculator

How the Internet Checksum Is Calculated

Adjacent bytes are interpreted as 16-bit words in network order. Those words are added using one’s-complement arithmetic. Any carry above bit 15 is added back into the low 16-bit result.

RFC 1071 gives the intermediate one’s-complement sum of this example as 0xDDF2. Complementing that value gives checksum 0x220D. :contentReference[oaicite:2]{index=2}

IPv4 Header Checksum Calculation

IPv4 includes a 16-bit Header Checksum field. RFC 791 specifies that this checksum covers the IPv4 header only, not the payload. Before calculating a new checksum, the checksum field itself is treated as zero. :contentReference[oaicite:3]{index=3}

Because fields such as Time To Live can change while a packet is forwarded, the IPv4 header checksum must be updated when relevant header data changes. RFC 791 explicitly notes that the header checksum is recomputed and verified as the header is processed. :contentReference[oaicite:4]{index=4}

Generate Checksum vs Verify Checksum

RFC 1071 describes checksum verification by summing the same set of octets including the checksum field. A valid result is all one bits in one’s-complement arithmetic. :contentReference[oaicite:5]{index=5}

Why Is End-Around Carry Required?

Ordinary binary addition discards a carry that exceeds the chosen word size. One’s-complement addition instead wraps that carry around and adds it back to the low-order bits.

This end-around carry behavior is fundamental to the Internet checksum and distinguishes it from ordinary unsigned addition.

What Happens with an Odd Number of Bytes?

The checksum operates on 16-bit words, so an odd number of input octets leaves one final unmatched byte.

For calculation purposes, that last byte is paired with a zero byte to form the final 16-bit word. The calculator reports when this zero padding was required.

Internet Checksum vs CRC

The Internet checksum is not a cyclic redundancy check. CRC algorithms use polynomial division, while the Internet checksum uses one’s-complement addition of 16-bit words.

The two mechanisms therefore have different mathematical properties and must not be substituted for each other simply because both are used for error detection.

IPv4 Checksum vs TCP and UDP Checksums

RFC 1071 specifically discusses the Internet checksum as used by IP, UDP, and TCP. :contentReference[oaicite:6]{index=6}

Why TCP and UDP Need a Pseudo-Header

A TCP or UDP checksum is not calculated over the transport segment alone. Protocol checksum construction also incorporates selected network-layer information through a pseudo-header.

That means simply pasting a TCP or UDP header into a generic Internet checksum calculator does not produce the complete wire checksum unless the required pseudo-header and payload bytes have also been included.

Internet Checksum Worked Example

This matches the numerical example described in RFC 1071. :contentReference[oaicite:7]{index=7}

Common Internet Checksum Mistakes

Internet Protocol Checksum Calculator FAQs