1828xbook.fm Page 477 Thursday, July 26, 2007 3:10 PM
IP Troubleshooting Tips and Tools
For the exam, you should check all the details documented in the question to determine the mask used by the various devices on the same LAN. Oftentimes, a question that is intended to test your knowledge will not just list all the information in a nice organized figure. Instead, you might have to look at the configuration and diagrams and use show commands to gather the information, and then apply the subnetting math explained in Chapter 12, “IP Addressing and Subnetting.” Figure 15-1 shows an example of a LAN that could be part of a test question. For convenience, the figure lists several details about IP addresses and masks, but for a given question, you might have to gather some of the facts from a figure, a simulator, and from an exhibit that lists command output. Figure 15-1
One LAN with Three Different Opinions About the Subnet 172.16.1.1/24 GW = 172.16.1.253
interface Fa0/0 ip address 172.16.1.253 255.255.255.128
PC1
Fa0/0 R1 Fa0/1 172.16.2.253
PC2
172.16.1.2/25 GW = 172.16.1.253
From the information in Figure 15-1, you can quickly tell that the two PCs use different masks (listed in prefix notation). In this case, you would need to know to look in the configuration for the subnet mask in the ip address interface subcommand, and then convert that mask to prefix notation to compare it with the other masks in this example. Table 15-3 lists the three differing opinions about the subnet. Table 15-3
Different Opinions About the Subnet in Figure 15-1 R1 Fa0/0
PC1
PC2
Mask
255.255.255.128
255.255.255.0
255.255.255.128
Subnet number
172.16.1.128
172.16.1.0
172.16.1.0
Broadcast address
172.16.1.255
172.16.1.255
172.16.1.127
In this case, several problem symptoms occur. For example, PC1 thinks 172.16.1.253 (R1) is in the same subnet, and PC1 thinks that it can forward packets to R1 over the LAN. However, R1 does not think that PC1 (172.16.1.1) is in the same subnet, so R1’s connected
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