IP Subnetting and Variable Length Subnet Masks (VLSMs)
IP Address Problem 1
S0/0 Lab B DCE _
You ask the client to go through the four troubleshooting steps that we learned about in the above section. Steps 1 through 3 work, but step 4 fails. By looking at the figure, can you deter- mine the problem? Look for clues in the network drawing. First, the WAN link between the Lab_A router and the Lab_B router shows the mask as a /27. You should already know this mask is 255.255.255.224 and then determine that all networks are using this mask. The net- work address is 192.168.1.0. What are our valid subnets and hosts? 256 – 224 = 32, so this makes our subnets 32, 64, 96, 128, etc. So, by looking at the figure, you can see that subnet 32 is being used by the Sales department, the WAN link is using subnet 96, and the Marketing department is using subnet 64.
Now you’ve got to determine what the valid host ranges are for each subnet. From what you learned at the beginning of this chapter, you should now be able to easily determine the subnet address, broadcast addresses, and valid host ranges. The valid hosts for the Sales LAN are 33 through 62—the broadcast address is 63 because the next subnet is 64, right? For the Marketing LAN, the valid hosts are 65 through 94 (broadcast 95), and for the WAN link, 97 through 126 (broadcast 127). By looking at the figure, you can determine that the default gateway on the Lab_B router is incorrect. That address is the broadcast address of the 64 subnet so there’s no way it could be a valid host.
Did you get all that? Maybe we should try another one, just to make sure. Figure 3.13 has a network problem. A user in the Sales LAN can’t get to ServerB. You have the user run through the four basic troubleshooting steps and find that the host can communicate to the local net- work, but not to the remote network. Find and define the IP addressing problem.
If you use the same steps used to solve the last problem, you can see first that the WAN link again provides the subnet mask to use— /29 or 255.255.255.248. You need to determine what the valid subnets, broadcast addresses, and valid host ranges are to solve this problem.