A fiield in a datagram, or IP packet header. This is an error correction field that ensures the integrity of header values. The checksum is formed by calculating a 16-bit one's complement checksum, including the original checksum. This must be recalculated at each router owing to the TTL. See RFC1141.

If no errors have occurred, the resulting checksum will be all ones. The checksum is not calculated for the encapsulated data.