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Troubleshooting Routing Tables

October 27th, 2006 · No Comments


At the foundation of TCP/IP networking is the routing table, a data construct on each host on a network3.jpgTCP/IP network. Routing tables serve the following three purposes:

* They are used to store the information about other subnets on the network and how you can reach hosts on these networks.
* They are used to determine which host (called the next-hop IP address) each packet should be forwarded to in order to reach the host this packet is ultimately destined for.
* They are used to determine which network interface (called the next-hop interface) should be used to forward this packet so it gets to its ultimate destination.

Understanding routing tables is therefore essential if you want to be able to effectively troubleshoot routing issues on a TCP/IP network. Let’s look at how routing tables work, what they look like in different scenarios, and what troubleshooting steps and tools might be indicated in different situations. We’ll begin by examining the routing table on a single-homed server (a server with a single network interface) that has a single IP address assigned to it. I’ve chosen this example because it’s the easiest one to understand, and in next month’s follow up article we’ll look at more complicated scenarios including servers with multiple IP addresses (such as web servers) and servers with multiple network interfaces (such as servers that are connected to both the LAN and to a separate network used for running backup jobs). TCP/IP Troubleshooting: A Structured Approach - Part 2: Troubleshooting Routing Tables

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