<P> An IP packet has no data checksum or any other footer after the data section . Typically the link layer encapsulates IP packets in frames with a CRC footer that detects most errors, and typically the end - to - end TCP layer checksum detects most other errors . </P> <P> The IPv4 packet header consists of 14 fields, of which 13 are required . The 14th field is optional and aptly named: options . The fields in the header are packed with the most significant byte first (big endian), and for the diagram and discussion, the most significant bits are considered to come first (MSB 0 bit numbering). The most significant bit is numbered 0, so the version field is actually found in the four most significant bits of the first byte, for example . </P> <Table> IPv4 Header Format <Tr> <Th> Offsets </Th> <Th> Octet </Th> <Th_colspan="8"> 0 </Th> <Th_colspan="8"> </Th> <Th_colspan="8"> </Th> <Th_colspan="8"> </Th> </Tr> <Tr> <Th> Octet </Th> <Th> Bit </Th> <Th> 0 </Th> <Th> </Th> <Th> </Th> <Th> </Th> <Th> </Th> <Th> 5 </Th> <Th> 6 </Th> <Th> 7 </Th> <Th> 8 </Th> <Th> 9 </Th> <Th> 10 </Th> <Th> 11 </Th> <Th> 12 </Th> <Th> 13 </Th> <Th> 14 </Th> <Th> 15 </Th> <Th> 16 </Th> <Th> 17 </Th> <Th> 18 </Th> <Th> 19 </Th> <Th> 20 </Th> <Th> 21 </Th> <Th> 22 </Th> <Th> 23 </Th> <Th> 24 </Th> <Th> 25 </Th> <Th> 26 </Th> <Th> 27 </Th> <Th> 28 </Th> <Th> 29 </Th> <Th> 30 </Th> <Th> 31 </Th> </Tr> <Tr> <Th> 0 </Th> <Th> 0 </Th> <Td_colspan="4"> Version </Td> <Td_colspan="4"> IHL </Td> <Td_colspan="6"> DSCP </Td> <Td_colspan="2"> ECN </Td> <Td_colspan="16"> Total Length </Td> </Tr> <Tr> <Th> </Th> <Th> 32 </Th> <Td_colspan="16"> Identification </Td> <Td_colspan="3"> Flags </Td> <Td_colspan="13"> Fragment Offset </Td> </Tr> <Tr> <Th> 8 </Th> <Th> 64 </Th> <Td_colspan="8"> Time To Live </Td> <Td_colspan="8"> Protocol </Td> <Td_colspan="16"> Header Checksum </Td> </Tr> <Tr> <Th> 12 </Th> <Th> 96 </Th> <Td_colspan="32"> Source IP Address </Td> </Tr> <Tr> <Th> 16 </Th> <Th> 128 </Th> <Td_colspan="32"> Destination IP Address </Td> </Tr> <Tr> <Th> 20 </Th> <Th> 160 </Th> <Td_colspan="32"> Options (if IHL> 5) </Td> </Tr> <Tr> <Th> 24 </Th> <Th> 192 </Th> </Tr> <Tr> <Th> 28 </Th> <Th> 224 </Th> </Tr> <Tr> <Th> 32 </Th> <Th> 256 </Th> </Tr> </Table> <Tr> <Th> Offsets </Th> <Th> Octet </Th> <Th_colspan="8"> 0 </Th> <Th_colspan="8"> </Th> <Th_colspan="8"> </Th> <Th_colspan="8"> </Th> </Tr>

Where is the source address located in an ipv4 header