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Chapter 2 - TCP/IP Communications

Cisco TCP/IP Routing Professional Reference
Chris Lewis
  Copyright © 1999 The McGraw-Hill Companies, Inc.

Putting It All Together
We have covered all the issues necessary to be able to explain how the TCP/IP protocols and reference tables (the routing and ARP tables) cooperate to transport a packet through an internetwork.
In the following explanation, we take the example of a PC trying to establish a Telnet session with a host machine that is located on the other side of a router.
Assuming the PC is running a Telnet client application, the command Telnet Sales-VAX is entered into the PC. By selecting the Telnet application, the destination port number is set to 23. Next, the hostname Sales-VAX needs to be resolved, meaning that its IP address needs to be found. This is done by either referencing Sales-VAX in the locally held hosts file, or requesting the IP address for Sales-VAX from a DNS server on the network.
Once the IP address has been determined, the PC looks to see if the destination IP address is on the same network (or subnet if netmasks are used). In this case it is not, so the PC will set the destination MAC address to that of its default gateway. The PC refers to its ARP table to determine the MAC address of the default gateway. If the IP address of the default gateway is not listed with a corresponding MAC address, the PC will issue an ARP broadcast to determine the MAC address. Once the MAC address of the default gateway is determined, all source and destination addresses are known and the packet is forwarded to the default gateway.
The default gateway will now receive the packet, examine the destination IP address and immediately look at its routing table. If the destination is on a directly connected segment, the default gateway will reference its ARP table, find the MAC address associated with the destination IP address of the Sales-VAX machine, and forward the packet to its final destination.
This process is re-examined in Chap. 3, when you will have a chance to see the previously described processes work in a Cisco router environment.

 


 
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