First, the foundations. The Internet Protocol provides a universal long-distance delivery mechanism on the third layer of the OSI model. It contains a set of parameters that were meant to be interpreted and eventually modified by intermediate systems. The header is shown in Figure 9-3.
Figure 9-3. The IP header structure
This is a four-bit value that is fixed to 4 (0100) in all IPv4 packets. IPv4 is the standard (and, in many cases, the only supported) layer three protocol over the Internet. Attempts to move toward a more advanced implementation, IPv6, have not been particularly successful so far—the author is willing ...