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Troubleshooting IP Addressing


the NIC card. This doesn’t mean that a cable is plugged into the NIC, only that the IP pro- tocol stack on the host can communicate to the NIC.

  • 3.

    From the DOS window, ping the default gateway (router). If the ping works, it means that the NIC is plugged into the network and can communicate on the local network. If it fails, then you have a local physical network problem that could be happening anywhere from the NIC to the router.

  • 4.

    If steps 1 through 3 were successful, try to ping the remote server. If that works, then you know that you have IP communication between the local host and the remote server. You also know that the remote physical network is working. If the user still can’t communicate with the server after steps 1 through 4 are successful, then

you probably have some type of name resolution problem, and need to check your Domain Name Server (DNS) settings. But if the ping to the remote server fails, then you know you have some type of remote physical network problem, and need to go to the server and work through steps 1 through 3 until you find the snag.

Once you’ve gone through all these steps, what do you do if you find a problem? How do you go about fixing an IP address configuration error? Let’s move on and discuss how to deter- mine the IP address problems and how to fix them.

Determining IP Address Problems

It’s common for a host, router, or other network device to be configured with the wrong IP address, subnet mask, or default gateway. Because this happens way too often, I’m going to teach you how to both determine and fix IP address configuration errors.

Once you’ve worked through the four basic steps of troubleshooting and determined there’s a problem, you obviously then need to find and fix it. It really helps to draw out the network and IP addressing scheme, unless you’re lucky and it’s already done. If so, go buy a lottery ticket, because though this should be done, it rarely is. And if it is, it’s usually outdated or inaccurate anyway, but typically it is not done and you’ll probably just have to bite the bullet and start from scratch.

I’ll show you how to draw out your network using CDP in Chapter 9: “Managing a Cisco Internetwork.”

Once you have your network accurately drawn out, including the IP addressing scheme, you then need to verify each host’s IP address, mask, and default gateway address to deter- mine the problem (I’m assuming you don’t have a physical problem, or if you did, you’ve already fixed it).

Let’s check out the example illustrated in Figure 3.12. A user in the Sales department calls and tells you that she can’t get to ServerA in the Marketing department. You ask her if she can get to ServerB in the Marketing department, but she doesn’t know because she doesn’t have rights to log on to that server. What do you do?

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