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Troubleshooting TCP/IP - Detailed
Steps
This article shows how to
troubleshoot TCP/IP connectivity between computers on a Windows
network. If you haven’t already done so, disable XP’s Internet
Connection Firewall on all local area network connections, and
remove all firewall programs on the network. Improperly
configured firewalls are the most common cause of TCP/IP
problems.
Open a Command Prompt Window
For many of these steps, you’ll be
typing at the command prompt. To open a command prompt window in
Windows 2000 or XP, click Start | Run, type cmd in the box, and
click OK. To open a command prompt window in Windows 95, 98, or
Me, click Start | Run, type command in the box, and click OK.
Type one command per line, and press Enter after each one to
execute it. To close the command prompt window, use the exit
command.
Determine the TCP/IP Settings
Determine
the TCP/IP settings of each computer on the local area network.
In XP, open the Network Connections folder, right click the LAN
connection, and click Status | Support | Details. For example,
here are the Status and Details views for the LAN connection on
an Internet Connection Sharing host.
In
Windows 95/98/Me, click Start | Run, type winipcfg in the box,
and click OK. Select the LAN adapter from the menu, and click
More Info. Here’s the winipcfg view for an ICS client running
Windows Me.
You can also see the TCP/IP settings from the command prompt.
This is especially convenient if a computer has more than one
network adapter. Use the ipconfig /all command, which is
available in all versions except Windows 95. The output from
this command can be long, so it’s best to write it to a file.
Specify the file name in the command this way:
ipconfig /all >ipconfig.txt
Here’s the output for a Windows XP ICS host that’s sharing its
cable modem connection:
Description of TCP/IP Settings
Here are the TCP/IP settings that are
used in network troubleshooting:
-
IP Address – Unique address
assigned to a network adapter. A computer with multiple
network adapters has an IP address for each one, and each one
must be in a different subnet.
-
Subnet Mask – Used in conjunction
with the IP address to determine which subnet an adapter
belongs to. At the simplest level, communication is only
possible between two network adapters when they’re in the same
subnet.
-
Default Gateway - IP address of a
computer or router, on one of this computer’s local area
networks, that knows how to communicate with subnets not
present on this computer. For an Internet connection, the
default gateway is a router belonging to your Internet service
provider, and all access to sites on the Internet goes through
it. For an ICS client, the default gateway is the ICS host. If
you use a hardware router, it serves as the default gateway.
-
DHCP Server – If an adapter is
configured to obtain an IP address automatically, this is the
address of the server that provides it. It could be your ISP,
an ICS host, or a hardware router.
-
DNS Servers – IP address of one or
more Domain Name Server computers. DNS servers translate
Internet names (like www.Far Reachtech.com) to their IP
addresses (like 65.77.232.100).
Subnets
If two computers are supposed to be on the same subnet, but
aren’t, something is wrong with the network hardware or software
configuration. This is most likely to happen when one of them
receives an IP address of 169.254.x.x, which indicates that:
-
It’s configured to obtain an IP
address automatically.
-
It couldn’t find a DHPC server on
the network to make the assignment.
-
Windows assigned it an Automatic
Private IP Address.
Pinging
The
ping command is the basic tool for testing TCP/IP connectivity.
It sends a special packet (called ICMP Echo) to a particular IP
address and looks for a reply. If everything is working right,
the reply comes back. If not, the ping times out in a few
seconds. By default, the ping command repeats the process four
times. Here’s an example of an ICS client computer pinging a
Windows XP Home Edition ICS host, using the host’s IP address
and its computer name.
When ping fails, you’ll see one of these error messages:
-
Request timed out - The IP address
is valid, but there’s no reply from it. If the IP address is
on a local area network, the most likely cause is a firewall
program blocking the ping.
-
Unknown host <name> or Ping request
could not find host <name> - The computer name doesn’t exist
on the local area network. Make sure that NetBIOS over TCP/IP
is enabled.
-
Destination host unreachable – The
IP address isn’t on a local area network, and the default
gateway can’t access it. Either there’s no default gateway,
its address is wrong, or it isn’t functioning.
Pinging the Local Area Network
Here is a series of ping commands to
use in finding where a problem occurs on a local area network.
Run them in the order shown, and don’t go on to the next command
until all of the previous commands work properly. In this
example:
-
The computer being tested is named
Winxp, with IP address 192.168.1.101.
-
There’s another computer on the
network, named Win98, with IP address 192.168.1.123
Substitute the appropriate IP
addresses and computer names for your network.
Command |
Target |
What Ping
Failure Indicates |
ping 127.0.0.1 |
Loopback address |
Corrupted TCP/IP installation |
ping localhost |
Loopback name |
Corrupted TCP/IP installation |
ping 192.168.1.101 |
This computer’s IP address |
Corrupted TCP/IP installation |
ping winxp |
This computer’s name |
Corrupted TCP/IP installation |
ping 192.168.1.123 |
Another computer’s IP address |
Bad hardware or NIC driver |
ping win98 |
Another computer’s name |
NetBIOS name resolution failure |
To fix a corrupted TCP/IP
Installation on Windows XP, follow the steps in this Microsoft
Knowledge Base article. For Windows 95/98/Me, un-install the
TCP/IP protocol in Control Panel | Network, reboot, and
re-install it. If that doesn’t fix it, use this procedure on
Windows 95 or 98.
Pinging the Internet
You can also use ping to find a
problem with Internet access. Run these commands in the order
shown, and don’t go on to the next command until all of the
previous commands work properly. Use the Default Gateway and DNS
Server addresses that you got from the winipcfg or ipconfig /all
command.
Command |
Target |
What Ping
Failure Indicates |
ping w.x.y.z |
Default Gateway |
Default Gateway down |
ping w.x.y.z |
DNS Server |
DNS Server down |
ping w.x.y.z |
Web site IP address |
Internet service provider or web site down |
ping www.something.com |
Web site name |
DNS Server down or |
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