CompTIA® Network+ Exam Notes : Troubleshooting Common Network Service Issues

5. Network Troubleshooting and Tools

5.5 Troubleshoot common network service issues

Incorrect gateway:

Scenario1: Two Servers Venus and Jupiter are connected together through a router. The existing configuration is as given below:

---------------------------------------------------
Venus:
IP address: 202.54.16.5
Subnet mask: 255.255.255.128
Default gateway:202.54.16.1
Jupiter:
IP address: 128.44.216.24
Subnet mask: 255.255.240.0
Default gateway: 202.54.16.1
Router:
IP address to network Venus: 202.54.16.1
IP address to network Jupiter: 128.44.216.1
-------------------------------------------------------

The servers are unable to communicate with each other. What is the most likely problem?

Solution: The default gateway on a host should be on the same subnet as that of the host. Otherwise, it is not reachable. For the subnet that Jupiter is residing, the default gateway is incorrectly given.

Scenario2: You are on your company's intranet running on TCP/IP. You are able to access the computers on the local subnet. But you are not able to access any computer on remote subnet. Which interface you need to suspect first to determine the problem?

Solution: Here, you are able to access the computers on the local subnet. However, you are not able to access the computers on remote subnets. Since you are not able to access any of the computers on the remote subnet, you need to ping the default gateway first. The default gateway might have been configured improperly on your computer.

Incorrect Netmask:

Scenario1: A Windows XP user complains that she cannot connect to any other computers on her local subnet even when using correct IP addresses. However, she can ping 127.0.0.1 and receive a reply.

Other users on her subnet do not report having any trouble. All client computers on the network are WINS-enabled. What is the most probable cause of the problem?

Solution: The most likely cause is that the subnet mask is not configured properly. If the TCP/IP stack is not installed properly, you will not be able to ping the local host (127.0.0.1). Also, even if the default gateway is wrongly configured, you will at least be able to ping the computers on the local subnet.

Default Gateway:

Scenario1: You have several computers on a network. All the computers are working on the same subnet. You have configured a router as one of the nodes and the router is connecting to your ISP. You find that none of your computers are able to browse the Internet, though they are able to access the other computers on the network. While diagnosing, you find that you are able to telnet to the router from any of the computers on the subnet, using the router's IP address. What is the likely cause?

Solution: Default Gateway on client computers can be assigned : Manually on each client Computer

To assign the default gateway manually on each client computer, you modify the TCP/IP Properties tab of ' Control Panel > Network' on each client computer. Through the use of DHCP Options

For assigning default gateway on client computers automatically, your NT Server should be running DHCP service. You configure the DHCP Options menu to enable default gateway.

When there are large number of client computers, it will be easier to configure the IP addresses and default gateway on client computers through the use of DHCP Service.

WINS (Windows Internet Name Server) is used for NetBIOS name resolution.

Since you are able to communicate with other computers, your NetBIOS name resolution OK. Also, you are able to Telnet, which precludes the possibility of TCP/IP protocol problems.

Scenario2: A network admin intends to build a router using a Linux computer for his two-subnet network. He has inserted two network cards, and connected one hub for each subnet. Workstations on a given subnet are able to communicate with each other. However, none of the workstations are able to communicate with any other workstation on the second subnet, and vice versa. All the workstations are able to ping respective router (Linux computer) interface, and communicate with the Linux computer. What is the likely problem?

Solution: Note that the workstations on a given subnet are able to communicate with each other, as well as with the Linux computer. Therefore, there is no problem with the networking portion of each subnet. It is likely that the IP protocol forwarding was not enabled on the Linux computer acting as the router.

Scenario3: A network of 10 computers is configured with physical star topology using a 12 port hub. But it is a bus configuration from logical point of view. The link to a computer in the 6th spot has broken. What are the consequences of the cable break?

Solution: The physical configuration is a star network. Therefore, all the computers except the computer on the broken network segment can communicate with each other.

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