Here we're not done yet, we have to apply now the last binary operation, which is the one's complement, and the result ( the checksum itself) will be:ī1E6 1011000111100110 <- The IP header checksum.
#Ip header checksum calculator plus
E188 1110000110001000 <-Fourth result plus next 16-bit word.ġ8D98 11000110110011000 <- here we see one odd bit ( carry), but we have to keep the checksum in "16-bit" words, so we add that odd bit to the result. Right, we need to sort all of these fields in 16-bit words and convert them into binary, so, it will be like this:Ġ000 0000000000000000 <- The checksum is set to zero. In simple terms, it means that we are in Transport Layer and the IP data. Now, the main ambiguity that arises that what is how can checksum be calculated on IP header as IP comes into the picture in the layer below the Transport Layer. the next four bytes (ac10 0a63) correspond to the Source IP address field, which is "172.16.10.99", and the next four bytes (ac10 0a0c) correspond to the Destination IP address field, which is "172.16.10.12". The CheckSum of the TCP is calculated by taking into account the TCP Header, TCP body and Pseudo IP header. When receiving, the calculation used is a different method. The next two bytes (B1E6) correspond to the IP header checksum of the packet, we'll calculate this "manually" later, so for us, this fields value is actually zero because we're gonna calculate it just as the "sender" did.
The next byte (06) correspond to the IP protocol field, which is set to 6, so the packet contains a TCP segment on it's payload. The next byte (40) correspond to the Time To Live field (TTL), which actually is 64 (4x16^1 + 0x16^0 = 64). Treating the flags field in 3-bit words, it's value is actually 4 (Don't Fragment), and the value for the fragment offset is obviously zero (000).
The next two bytes (4000) correspond to the flags and fragment offset IP header fields, which are divided in 3 bits for the flags and 13 for the fragment offset. The next two bytes (1C46) correspond to the Identification field, which in this packet is 7238 ( 1x16^3 + Cx16^2 + 4x16^1 + 6x16^0 = 7238).
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