Internet Core Protocols: The Definitive Guide

Local Versus Remote Delivery

The IP header stores the IP addresses of both the source and destination systems. If the destination system is on the same physical network as the sending system, then the sender will attempt to deliver the datagram directly to the recipient, as shown in Figure 2.3. In this model, the sender knows that the recipient is on the same local network, so it transmits the data directly to the recipient, using the low-level protocols appropriate for that network medium.

Figure 2.3. An example of local delivery

However, if the two systems are not connected to the same IP network, then the sender must find another node on the local network that is able to relay the IP datagram on to its final destination. This intermediate system would then have to deliver the datagram if the final recipient was directly accessible, or it would have to send the datagram on to yet another intermediary system for subsequent delivery. Eventually, the datagram would get to the destination system.

A slightly more complex representation of this can be seen in Figure 2.4. In that example, the sending system knows that the destination system is on a remote network, so it locates an intermediate system that can forward the data on to the final destination. It then locates the hardware address of the forwarding system, and passes the data to the intermediate system using the low-level protocols ...