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Troubleshooting IP Addressing

145

FIGURE 3.13

IP Address Problem 2

ServerA

ServerB

192.168.1.25

192.168.1.86

192.168.1.87

Default gateway:

Default gateway:

Default gateway:

192.168.1.30

192.168.1.81

192.168.1.81

Sales

Marketing

F0/27

1900

F0/26

F0/0

192.168.1.30

S0/1 DCE

F0/2

2950

F0/3

F0/1

F0/0

192.168.1.81

Lab_A

S0/0

192.168.1.41/29

S0/0 Lab B DCE _

192.168.1.46/29

The 248 mask is a block size of 8 (256 – 248 = 8), so the subnets both start and increment in multiples of 8. By looking at the figure, the Sales LAN is in the 24 subnet, the WAN is in the 40 subnet, and the Marketing LAN is in the 80 subnet. Can you see the problem yet? The valid host range for the Sales LAN is 25–30, and the configuration appears correct. The valid host range for the WAN link is 41–46, and this also appears correct. The valid host range for the 80 subnet is 81–86, with a broadcast address of 87 because the next subnet is 88. ServerB has been configured with the broadcast address of the subnet.

OK, now that you can figure out misconfigured IP addresses on hosts, what do you do if a host doesn’t have an IP address and you need to assign one? What you need to do is look at other hosts on the LAN and figure out the network, mask, and default gateway. Let’s take a look at a couple of examples of how to find and apply valid IP addresses to hosts.

If you need to assign a server and router IP addresses on a LAN and the subnet assigned on that segment is 192.168.20.24/29, and the router needs to be assigned the first usable address and the server the last valid host ID, what are the IP address, mask, and default gateway assigned to the server?

To answer this, you must know that a /29 is a 255.255.255.248 mask, which provides a block size of 8. The subnet is known as 24, the next subnet in a block of 8 is 32, so the broadcast address of the 24 subnet is 31, which makes the valid host range 25–30.

Server IP address: 192.168.20.30 Server mask: 255.255.255.248 Default gateway: 192.168.20.25 (router’s IP address) As another example, let’s take a look at Figure 3.14 and solve this problem.

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