Its designers considered that the whole-packet link layer checksumming provided in protocols, such as PPP and Ethernet, combined with the use of checksums in upper layer protocols such as TCP and UDP, are sufficient.
The Internet checksum is mandatory to detect errors in IPv6 UDP packets (including data payload).
The Internet checksum is used to detect errors in ICMP packets (including data payload).
The router must adjust the checksum if it changes the IP header (such as when decrementing the TTL).
Initial addition: 4500 + 0073 + 0000 + 4000 + 4011 + c0a8 + 0001 + c0a8 + 00c7 = 2479c Carry addition is then made by adding the fifth hexadecimal digit to the first 4 digits: 2 + 479c = 479e The checksum is then the ones' complement (bitwise NOT) of this result: NOT 479e = b861 This checksum value is shown as underlined in the original IP packet header above.