The first step to learning about troubleshooting falls into the
theoretical arena with the OSI model. See this table for a list of the OSI model layers and
their names. The OSI model helps us break networks into different layers
in terms of how they correspond to the different applications and
protocols we use to transmit data. Each protocol works in different ways,
and the OSI model helps us understand how they function and what they
do.1. Using the Network Diagnostics and Repair Option
Windows 7 offers the ability to self-diagnose network
connectivity problems using Windows Network Diagnostics. This feature
can help to identify problems with network adapters and TCP/IP issues.
If you do not have a cable properly connecting the adapter to the
network, it will also notify you of this problem. The wizard goes
through a simple set of tests to determine the problem, alleviating the
need to complete these tasks manually. However, if the wizard cannot fix
the problem, you must determine the problem yourself, as discussed in
the other troubleshooting sections of this chapter.
You can diagnose and repair network problems by completing these
steps:
Click Start, and then click Control Panel.
In the Control Panel, click Network and Internet→Network and
Sharing Center.
In the Network and Sharing Center, click “Change adapter
settings.” This opens the Network Connections window.
Right-click the network connection you want to troubleshoot
and then select Diagnose.
When Windows Network Diagnostics finishes testing your network
configuration, you’ll see a list of possible solutions, as shown in
Figure 1. Follow the
instructions provided to try to correct the problem, or click a
solution to have Windows Network Diagnostics perform a
troubleshooting task for you.
2. Checking Physical Connectivity
Using the functionality of the OSI model, you can identify where
protocols function to aid in troubleshooting network problems. If you
are unable to connect to a network, you should begin at the layer 1
level. This layer tells you to look specifically at the physical
connectivity of your computer to the network. Identifying whether your
cabling works correctly should be the first step of network
troubleshooting. You can usually verify that you have connectivity by
making sure that the link light is lit on the network adapter. Usually
if your link light is lit and the activity light is showing activity by
flashing, you can assume the computer has connectivity. With wireless
routers, you can go a step further by logging in to the router and
confirming whether the device shows up as being currently
connected.
If you complete the troubleshooting steps in the section but you
still have issues connecting to the network properly, replace the
network cable or check the wireless configuration settings. If you have
doubts as to whether you have connectivity with Ethernet, take a few minutes to replace
the network cable, as it is easy and inexpensive to replace this piece
of equipment. Telltale signs that you need to replace your cable include
no link light when connected to a powered-on network device, and the
fact that you can either only send or only receive data.
If you have verified that you have connectivity, you must
determine whether the network adapter exists within Device Manager on
your computer. If your computer sees the network adapter, verify that
you have the latest driver installed. Sometimes you can have problems
with network functionality due to an old device driver associated with
the network adapter. This scenario can happen after a software update to
the operating system and when a third-party application makes changes to
a shared control file, or to a file the driver relies upon for quality
communications.
If you still cannot connect to the network, check your TCP/IP
configuration settings.
You can also use the techniques discussed in the upcoming sections Section 3 and Section 4 to help you with troubleshooting
configuration issues. If none of these efforts resolves the problem, try
replacing the network adapter with a second network adapter. This should
verify connectivity problems or resolve the issue. If the problem
follows the adapter, you can assume the adapter has a problem. If you
still cannot connect to the network with a new adapter, verify that the
slot in the motherboard works correctly. If your network adapters are
integrated into the motherboard, you can still add a different physical
network adapter into a slot on the motherboard. You may then want to
disable the integrated adapters in firmware to avoid IRQ conflicts on
your computer.
To check the functionality of the slot in the motherboard, install
the network adapter into a different slot on the motherboard. If you
still cannot connect to the network or see an adapter, you should update
firmware on your motherboard or contact the manufacturer’s technical
support to either identify the problem with the board or get a
replacement board, assuming you have warranty support on the board in
question. Once you have the replacement board, you can connect the
network adapter to verify connectivity.
3. Using the Command Line to Diagnose Network Problems
Microsoft offers many different tools to help diagnose
network problems. The best tools for testing your network are those
available at the command line. For troubleshooting, be sure to start the
command line with elevated privileges by completing the following
steps:
Click Start, click All Programs, and then click
Accessories.
Right-click Command Prompt and then select “Run as
administrator.”
This opens an administrator command prompt that you can use to
perform any necessary troubleshooting procedures.
To begin troubleshooting, you should first determine the IP
configuration for all network adapters on your computer. To accomplish
this task, type IPconfig /all at the
command prompt you opened previously. You will see the output of the
TCP/IP stack as well as some physical details concerning the
characteristics of your local machine, similar to Example 1, which gives you a great view
of the properties controlling access to your network and its resources.
You can see immediately the name of the host, type of connection,
routing capability, DNS name, MAC address, IP address, DHCP server IP
address, subnet, default gateway, and IP address for each adapter
connected to the computer.
Example 1. IPConfig /all output
C:\>IPconfig /all
Windows IP Configuration
Host Name . . . . . . . . . . . . : RC1-5600
Primary Dns Suffix . . . . . . . :
Node Type . . . . . . . . . . . . : Hybrid
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
DNS Suffix Search List. . . . . . : globalsuite.net
Wireless LAN adapter Wireless Network Connection:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . : ok.cox.net
Description . . . . . . . . . . . : Broadcom 802.11g Network Adapter
Physical Address. . . . . . . . . : 00-14-A5-A0-15-F1
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . : globalsuite.net
Description . . . . . . . . . . . : Realtek RTL8139/810x Fast Ethernet NIC
Physical Address. . . . . . . . . : 00-16-36-46-FF-15
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::2da2:3d:9f2e:297c%10(Preferred)
IPv4 Address. . . . . . . . . . . : 158.18.184.133(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Lease Obtained. . . . . . . . . . : Tuesday, October 12, 2009 2:02:32 PM
Lease Expires . . . . . . . . . . : Wednesday, October 13, 2009 2:35:32 PM
Default Gateway . . . . . . . . . : 158.18.0.1
DHCP Server . . . . . . . . . . . : 158.18.0.1
DHCPv6 IAID . . . . . . . . . . . : 234886710
DNS Servers . . . . . . . . . . . : 4.2.2.1
NetBIOS over Tcpip. . . . . . . . : Enabled
|
The first setting you need to verify is the IP address of the
computer. If you are using DHCP on a network device, make sure you see
DHCP Enabled. . . . . : Yes in the
output of the IPCONFIG /ALL command.
If you see this setting, verify that you have received an IP address. If
you do not have an IP address, check whether you can see an address for
the DHCP server. When identifying the IP address, if you see either
0.0.0.0 or 169.265.X.X as an IP address, you did not connect to the DHCP
server. You can try to reapply for an IP address by typing the IPconfig /renew command at the command line.
If this fails to give you an IP address, try inputting a static IP
address within the subnet of the DHCP server to see if you can gain
access to the network . If you can, make sure the DHCP server is running correctly.
Alternatively, simply unplug and then plug in the device to force it to
reset itself.
Windows offers a host of commands for testing your network, but by
far the king of all commands is ping. Pinging allows
you to direct specific-size packets at a computer to verify
connectivity. Ping actually echoes
back with information on the connectivity to another computer. If you
wanted to verify your ability to send packets, you should first ping
your computer on the local loopback address, 127.0.0.1. The loopback
address provides a simple mechanism for testing the Windows network
stack.
Next, proceed to verify that name resolution is working correctly.
You can accomplish this task by pinging the name of a computer on the
Internet, such as a website. If you are unable to ping the computers on
the Internet, check the TCP/IP settings of the network adapter to verify
that you have enabled the TCP/IP protocol. See Example 2 for examples of how to ping different network
hosts by name or IP address.
Example 2. Ping command
C:\>ping 127.0.0.1
Pinging 127.0.0.1 with 32 bytes of data:
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Reply from 127.0.0.1: bytes=32 time<1ms TTL=128
Ping statistics for 127.0.0.1:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milliseconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
C:\>ping google.com
Pinging google.com [72.14.207.99] with 32 bytes of data:
Reply from 72.14.207.99: bytes=32 time=52ms TTL=235
Reply from 72.14.207.99: bytes=32 time=75ms TTL=235
Reply from 72.14.207.99: bytes=32 time=51ms TTL=235
Reply from 72.14.207.99: bytes=32 time=52ms TTL=235
Ping statistics for 72.14.207.99:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milliseconds:
Minimum = 51ms, Maximum = 75ms, Average = 57ms
|
If you cannot receive packets by pinging another computer, you
should proceed to ping the IP address of your default gateway. You can
find the IP address of your default gateway in the output of the
IPconfig /all command.
If you are unable to ping your default gateway, either contact
your network administrator or check the gateway. In most instances, your
gateway consists of a broadband router or similar device, and you should
check the device for proper functionality. You can identify errors with
packets on the device by looking for status lights identifying packet
collisions or another error. Try resetting the device by unplugging it,
waiting a few seconds, and powering it on again. Alternatively, if the
device has a Reset button, you can press this button as well. If this
process does not work, you may need to update firmware on or replace the
network device. If your default gateway consists of a cable modem or DSL
router, contact your service provider for steps to alleviate your
problem.
If you can ping your default gateway but are unable to connect to
the Internet and you are using a network router or other personally
managed network device for a gateway, check the cable going to your
outside provider. You should also try to ping outside your internal
network to test for outside connectivity. If you can successfully ping
an external IP address your gateway should be in good shape. If you can
ping by IP address externally but not to a name, such as Google.com, you need
to verify that you have input the IP address of your DNS server properly
in the TCP/IP configuration. With DHCP, the network device usually
provides the DNS server address, so you would need to check the
configuration of the device. If you verify these settings but still
cannot connect to the Internet, use the NSLookup command to
check for DNS resolution. See Example 3 for an
example of how to use the NSLookup
command.
Example 3. NSLookup output
C:\>nslookup
Default Server: ns1.securestream.net
Address: 69.150.220.8
> securestream.net
Server: ns1.securestream.net
Address: 69.150.220.8
Name: securestream.net
Address: 69.150.220.9
|
To verify DNS resolution for connectivity to the Internet or
possibly your Active Directory domain, type NSLookup at the command line. You should
receive a > prompt. Type the name
of the domain to which you want to connect and press the Enter key. If
you receive output showing the IP addresses of the domain, your
resolution works correctly. If you receive output that says something
like “Non-Existent” domain, you should try another DNS server for output
or contact your ISP to find out why its DNS server fails queries.
If you can query the DNS name correctly but are still having
problems with Internet connectivity, use the tracetrt command to check the routing device
hops between yourself and the desired location. At the command line, you
can accomplish this task by typing:
tracert <Destination IP or Name>
You will see a maximum of 30 hops to the destination, including an
IP address, and possibly the name of the device. If you see timeouts or
other error messages for addresses outside your network, there are
problems outside of your control. You should contact your service
provider to determine whether it is aware of these problems and is
working to correct them.
4. Fixing Network Problems
TCP/IP networking has many different facets, and additional
protocols that ride on top of it. Although it is the single most
prolific technology in use today, there are inherent problems with some
implementations. Use Table 1 to help
diagnose common problems with network connectivity or protocol
issues.
Table 1. Troubleshooting matrix
| Problem | Resolution |
|---|
| No IP
Address | Check DHCP scope on the
router or server.
Input a static IP
address. |
| Cannot Ping
Machine | Ping
<Host Name>.
Ping
<Default
Gateway>. |
| Cannot Ping
127.0.0.1 | Verify that the TCP/IP
protocols are enabled on the adapter.
Verify that
the adapter is enabled in Device Manager. |
| Cannot Ping Default
Gateway | Verify that the gateway
has network connectivity.
Check the cables
connecting your computer to the network device.
Reset the network device by powering it off and then
powering it on.
Contact your ISP.
Replace the network device. |
| Cannot Reach Internet | Check gateway
connectivity using TRACERT
<IP Destination>.
Verify DNS resolution using NSLookup.
Input DNS
server addresses as part of the network device DHCP
configuration.
Input DNS server addresses in the
TCP/IP properties of the network adapter. |
| No Network Connectivity | For Ethernet, check the
cable connecting the network adapter to the network.
For wireless, reorient antenna, relocate router, or try a
different channel.
Check Link/Activity status on
the adapter.
Update the network adapter
driver.
Flash firmware on motherboard (integrated
adapters only).
Move the network adapter to a
different slot on the motherboard.
Replace the
network adapter. |
| Only Send or Receive
Packets | For Ethernet, reseat the
network cable or replace the network cable. For wireless,
reorient antennae, relocate router, or try a different
channel. |
