Welcome to Certifiable, your exam prep headquarters. Here you'll find questions about some of the tricky areas that are fair game for the certification exams. Following the questions, you'll find the correct answers and explanatory text. We change the questions biweekly.
Questions (November 3, 2000)
The following questions are simple versions of the troubleshooting/scenario questions you'll find on the certification exams. In the next few issues, I'll include more difficult questions that tackle more complicated scenarios. All three of the following questions cover network-related problems, so use the bottom-up approach to finding the answer (see the Commentary above). This strategy works well in just about any troubleshooting situation, but it works especially well for networking-related problems because of the Open System Interconnection (OSI) model's design. Once you are familiar with the services each layer performs, you should be able to make sense of the information that tools such as ping, ipconfig, nslookup, and arp give you.
Lest you think these are "made-up" scenarios, let me assure you that Question 3 comes directly from something that happened to me just last week. Given that the network was perfectly fine the previous week, a combination of isolating what changed over the weekend and analyzing the output of ipconfig gave us the cause the problem, which in turn provided the solution. The problem may seem simple once you see the answer, but when you have more than 100 people calling on a Monday morning before you've had your coffee, it doesn't seem so simple at all!
After assigning manual TCP/IP addresses to workstations, the administrator notices that one workstation can't see the other computers in Network Neighborhood. Running ipconfig /all on the machine shows that the IP address is 0.0.0.0. What is causing the problem?
- The workstation's IP address is already in use.
- The administrator entered the wrong information.
- The machine hasn't fully initialized the IP address.
- The network is down.
- The cable is unplugged from the machine.
A computer can't access a server with an IP address of 10.56.78.66. The computer can, however, successfully ping another computer with an IP address of 10.56.78.97. When you run ipconfig, you receive the following information:
Windows NT IP Configuration: Ethernet adapter NDISGENERIC1: IP address. . . . . . . : 10.56.78.98 SubNet Mask . . . . . . : 255.255.255.240 Default Gateway . . . . :
How would you correct the problem?
- Move the computer to the same subnet as the server.
- Correct the IP address.
- Enter a Default Gateway value.
- Change the Subnet Mask.
At 8:00 A.M. on a Monday, you and your staff begin receiving calls from employees who tell you that they can't access the Internet or see the email server on the network. Upon further investigation, you notice that these employees' computers show some other employees' computers in Network Neighborhood, but none of the employees can see computers on subnets other than their own.
All computers on the network are configured to use DHCP. Your router is configured to forward BootP broadcasts, and all subnets use the same DHCP server (192.168.1.2), which has scopes with network addresses from 192.168.1.0 to 192.168.15.0. The subnet mask is 255.255.0.0 and the default gateway is 192.168.x.1, where x is equal to the value in the local subnet's third octet. All computers on the network use the same WINS server (192.168.1.3). From an employee computer, you can ping 192.168.0.2 but not 192.168.1.2.
When you run ipconfig /all on the computers with problems, ipconfig reports the following information:
DHCP enabled: yes Host address: 192.168.0.x Subnet Mask: 255.255.255.0 Default Gateway: 192.168.0.1 DHCP server: 192.168.0.2 WINS server: 192.168.0.3
Which of the following reasons might explain why the employees' computers can't communicate with other computers on the network. (Choose two.)
- The computers are using Automatic Client Configuration to get an IP address.
- Someone has installed the DHCP service on a computer on the network.
- The subnet mask is incorrect.
- A server is running the DHCP Relay Agent.
- The WINS server setting is incorrect.
- The DHCP server is incorrect.
Answer to Question 1
The correct answer is A— The workstation's IP address is already in use.
IP communications fail if a network adapter has been assigned an IP address already in use on the network. When the network stack is initialized, an error message appears informing the user that the address is already in use and that the TCP/IP configuration will be disabled. Ipconfig shows an address of 0.0.0.0 for enabled adapters that have an invalid IP address assigned.
Answer to Question 2
The correct answer is C— Enter a Default Gateway value.
A TCP/IP address consists of two parts, the Network ID and the Host ID. The system uses the subnet mask to determine whether the destination computer's address is on the same network as the source address or somewhere else. In the subnet mask 255.255.255.240, the first 28 bits of the address identify the Network ID and the last four bits indicate the Host ID. In this example, 10.56.78.66 is on a different subnet, and you need to enter the IP address of a router (i.e., the default gateway) that can send the network packets to the proper subnetwork.
Answer to Question 3
The correct answers are B— Someone has installed the DHCP service on a computer on the network; and D— A server is running the DHCP Relay Agent.
One way to answer this question is to eliminate what can't be causing the problems. Answer A is incorrect because Automatic Client Configuration uses the network address 169.254.0.0 with a subnet mask of 255.255.0.0. Answer C is incorrect because the subnet mask is set when the client receives the IP address. Answer E is incorrect because WINS doesn't assign IP addresses. Answer F is incorrect because the DHCP server's IP address is set when the client receives the address; you can't manually set the DHCP server's address.
The other way to determine the problem is to recognize that the client computers—which are configured as a DHCP clients— have a network address that doesn't correspond to one of the network addresses that the DHCP server offers. The only way the computers could be receiving an IP address from the 192.168.0.0 network is if another DHCP server is answering the DHCP client broadcast messages. Resolving this problem requires finding the new DHCP server.