THE IP HEADER
The information in the IP datagram is used by the forwarding protocol at the router to determine the “next node.” This information is contained in selected fields in the header of the IP datagram. The router uses the IP header and the routing table to “glue” together the seemingly disparate Internet. To see how, we take a look at the information in the IP header, and Figure 2-12 provides an illustration of the IP datagram. Some of the fields in the IP datagram are not relevant to our analysis, but I will describe them for the sake of giving you a complete picture of the overall IP operations.
Figure 2-12. The IP diagram