Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 138
Datagram Routing

Now that routing fundamentals, the RIP and OSPF protocols, and routing tables have been discussed, the slide shows a routed packet using direct and indirect routing. This is regardless of the routing protocol. In this slide, we can see that a PC (endstation A) is trying to pass a datagram to a host machine, called host D. The host machine is one hop (one router) away. The IP layer of the PC (endstation A) knows that it must use a router (the source and destination network addresses are different), and will use RIP or the default gateway to determine the IP address of the router to use. Upon determining the router’s physical address, it will physically (MAC layer) address the packet to the router at port B. The source and destination IP addresses in the IP header of this datagram will be the PC as the source, and the destination IP address as the host. The source (PC) and final destination (the host) IP addresses will be embedded into the IP header and will not change throughout the routing of this datagram.

Datagram Routing

The router will receive this packet and extract the network number from the final destination IP address in the received IP header. The physical address headers will be stripped. The extracted network number will be compared to the router’s internal routing table.

The router will determine that the destination network can be reached directly through one of its ports (the destination network is directly attached). The router will determine the destination station’s physical address through its ARP table (or it may request it through the ARP process). The router will then build a packet with the original datagram sent by endstation A to submit to host D. The physical source address will be the router’s, the physical destination address will be host D’s. The packet is then transmitted to host D.

Notice throughout this that only the MAC addresses changed; the IP addresses in the IP header stayed the same. The router will change the TTL and the CRC in the IP header, but that is the only thing that changes in the IP header.