Internet Core Protocols: The Definitive Guide

The Internet Protocol

When an IP-enabled device wants to send data to another IP node, the data-link services on that device convert the IP datagrams into a format usable by the local network medium, and then send the data to the destination system using the addressing and framing mechanisms dictated by the network.

These steps occur on each of the networks that an IP datagram traverses on its way to the final destination system. If an IP datagram were sent from a dial-up user working at her home in Los Angeles to a server in Rome, Italy, the number of networks that would be crossed could be quite high. But at each step of the way, the data would be transmitted using the low-level protocols appropriate for each of the particular networks being crossed.

In this regard, IP provides a virtual representation of the global Internet to the hosts that are on it. IP provides a datagram formatting and addressing mechanism that is not dependent upon any of the specific characteristics of the individual networks that make up the global Internet. Data can be sent to an IP address, and the data will be encapsulated and transmitted according to the rules of each of the intermediary networks, with the IP datagram being used to provide delivery clues to the sending, receiving, and intermediary devices. Essentially, routing occurs at the network layer (IP), while delivery occurs at the data-link layer (Ethernet, modems, whatever).

This concept is illustrated in Figure 1.8. In that example, data sent ...