Understanding Internet Protocol Addressing
Figure A.1 shows the layout of a typical IP packet. You should note that we have already discussed the TTL value. This field is used to keep an IP packet from circulating in an endless loop indefinitely when one or more routers have been misconfigured. The other important fields (perhaps the most important) are the source and destination address fields. The remainder of the IP packet is shown for completeness so that you can see how data is structured as it makes its way through the Internet.
Figure A.1. The IP packet consists of many fields, but the addressing fields are of particular concern here.
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