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Article ID: 169790 - Last Review: January 19, 2007 - Revision: 2.4

This article was previously published under Q169790

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SUMMARY

This article describes how to troubleshoot some common network communication problems you may experience when you use TCP/IP as your network protocol. These problems usually fall into one of the following two categories:
  • You are unable to connect to a specific IP address.

  • You are unable to connect to a specific host or NetBIOS name.
If you are unable to connect to a specific IP address, the problem is related to basic connectivity. If you are able to connect to a specific IP address, but you are not able to connect using the host or NetBIOS name for that IP address, the problem is related to name resolution.

NOTE: All of the troubleshooting steps below will work on the NT and 2000 platforms, but may not work on the Win9x (except Win ME) platforms. However, the basic diagnostic and troubleshooting methods are the same for all of these Windows operating systems.

MORE INFORMATION

To determine whether the problem is related to basic connectivity or name resolution, use the following procedure to determine whether you can connect to a specific IP address.

Connect to an IP Address

Try to connect to another computer on your network using its IP address and the TCP/IP program or utility of your choice. Web browsers, ftp, and Telnet are some programs and utilities commonly used to connect to other computers with TCP/IP.

NOTE: If you do not know the IP address for your Windows NT or 2000 computer that you are attempting to connect to, run the IPCONFIG /ALL command at a command prompt on the other computer.

If you cannot connect to the other computer using its IP address, there is a basic connectivity problem. Use the information in the "Unable to Connect to a Specific IP Address" section below to resolve your issue. If you can connect to the other computer using its IP address, but you are not able to connect using the host or NetBIOS name of the other computuer, there is probably a name resolution problem. Use the information in the "Unable to Connect to a Specific Host or NetBIOS Name" section below to resolve your issue.

UNABLE TO CONNECT TO A SPECIFIC IP ADDRESS

Follow the procedures in each of the following sections in order. After you finish each procedure, check to see if you can connect to the other computer using its IP address.

Check Your TCP/IP Configuration

When you use TCP/IP as your network protocol, an incorrect TCP/IP setting (such as an incorrect IP address or an incorrect subnet mask) can cause communication problems. To determine whether Windows NT or 2000 has recorded an error due to an incorrect TCP/IP setting, examine the Event Viewer system log and look for any entry with TCP/IP or DHCP as the source. To read an Event Viewer entry, double-click the entry.

NOTE: If Event Viewer records a DHCP error, you should report the error to your network administrator.

If you receive TCP/IP errors in the Event Viewer system log, resolve each error received as indicated by the error message. For example, if you receive an error stating that the IP address parameter is incorrect, you should verify that your IP address is valid.

If there are no errors in the Event Viewer system log, follow these steps to make sure that the correct TCP/IP configuration information is being used:
  1. Use the IPCONFIG command to determine your computer's basic TCP/IP settings. To do so, type ipconfig at a command prompt.
  2. Verify that the IP address and subnet mask displayed by the IPCONFIG command are the correct values for your computer. If you are not sure what the correct values are, contact your network administrator.

Ping the Loopback Address

Use the PING command to verify that TCP/IP is working properly. To do so, ping the loopback address (127.0.0.1) by typing the following command at a command prompt:
ping 127.0.0.1
You should receive a response similar to the following:
   Pinging 127.0.0.1 with 32 bytes of data:

   Reply from 127.0.0.1: bytes=32 time=<10ms TTL=128
   Reply from 127.0.0.1: bytes=32 time=<10ms TTL=128
   Reply from 127.0.0.1: bytes=32 time=<10ms TTL=128
   Reply from 127.0.0.1: bytes=32 time=<10ms TTL=128
				
If you receive an error message at this point, TCP/IP is not properly installed. To remove and reinstall TCP/IP, follow these steps:

NOTE: You must be logged on as a user with Administrator rights to complete these steps.
  1. In Control Panel, double-click Network, and then click the Protocols tab.
  2. Click TCP/IP Protocol to select it, click Remove, and then click Yes.
  3. Click Close, and then click Yes to restart the computer.
  4. Log on as a user with Administrator rights.
  5. In Control Panel, double-click Network, and then click the Protocols tab.
  6. Click Add, click TCP/IP Protocol to select it, and then click OK.
  7. If you want to use DHCP, click Yes when prompted. If you do not want to use DHCP, click No.
  8. When prompted, type the path for the Windows NT source files, click Continue, and then click Close.
  9. If you are not using DHCP, you are prompted for your TCP/IP configuration information. Provide the appropriate values, and then click OK. If you are not sure what the appropriate values are, contact your network administrator.
  10. Click No when you are prompted to restart your computer. If you have installed a Windows NT Service Pack, you need to reinstall the Service Pack before you restart your computer.
  11. Restart your computer.
If you receive an error message while removing and reinstalling TCP/IP, you may need to manually remove TCP/IP from the Windows NT registry. For information about manually removing TCP/IP from the Windows NT registry, see the following article in the Microsoft Knowledge Base:
151237  (http://support.microsoft.com/kb/151237/EN-US/ ) Error Message When Installing TCP/IP or Adding TCP/IP Service

Ping Your Computer's IP Address

If you can ping the loopback address successfully, attempt to ping your own IP address by typing ping <IP address> at a command prompt, where <IP address> is your computer's IP address.

NOTE: If you do not know your computer's IP address, you can obtain that information by typing ipconfig at a command prompt.

You should receive a response similar to the following:
   Pinging <###.###.###.###> with 32 bytes of data:

   Reply from <###.###.###.###>: bytes=32 time=77ms TTL=28
   Reply from <###.###.###.###>: bytes=32 time=80ms TTL=28
   Reply from <###.###.###.###>: bytes=32 time=78ms TTL=28
   Reply from <###.###.###.###>: bytes=32 time=79ms TTL=28
				
Where <###.###.###.###> is your computer's IP address.

If you receive an error message at this point, there may be a communication problem between Windows NT and your network adapter. To correct this problem, remove and reinstall your network adapter driver. To do so, follow these steps:

NOTE: You must be logged on as a user with Administrator rights to complete these steps.
  1. In Control Panel, double-click Network, and then click the Adapters tab.
  2. Click your network adapter driver to select it, click Remove, and then click Yes.
  3. Click Close, and then click Yes to restart your computer.
  4. Log on as a user with Administrator rights.
  5. In Control Panel, double-click Network, and then click the Adapters tab.
  6. Click Add, click your network adapter driver to select it, and then click OK.
  7. Use the dialog box(es) provided to configure your network adapter, and then click OK.
  8. When prompted, type the path for the Windows NT source files, click Continue, and then click Close.
  9. When you are prompted for your TCP/IP configuration information, provide the appropriate values, and then click OK. If you are not sure what the appropriate values are, contact your network administrator.
  10. Click No when you are prompted to restart your computer. If you have installed a Windows NT Service Pack, you need to reinstall the Service Pack before you restart your computer.
  11. Restart your computer.
If you are unable to ping your computer's IP address after removing and reinstalling your network adapter driver, contact the manufacturer of your network adapter to verify that you are using the appropriate Windows NT driver for your network adapter.

Clear the Address Resolution Protocol (ARP) Cache

The address resolution protocol (ARP) cache is a list of recently resolved IP address to Media Access Control (MAC) address mappings. The MAC address is the unique physical address embedded in each network adapter.

If an entry in the ARP cache is incorrect, IP datagrams may be sent to the wrong computer. To display all mappings currently in the ARP cache, use the ARP command by typing arp -a at a command prompt. You should receive either a "No ARP Entries Found" message (if the ARP cache is empty) or a response similar to the following:
   Interface: 10.1.1.3 on Interface 2
   Internet Address      Physical Address      Type
   10.1.1.7              08-00-02-06-ed-20     dynamic
   10.1.1.254            08-00-02-0a-a3-10     dynamic
				
To remove any incorrect entries in the ARP cache, clear all entries using the following command:
arp -d <IP address>
Where <IP address> is an Internet address stored in the ARP cache. Use this command for each entry in the ARP cache until all entries have been deleted.

For more information on the syntax, options, and usage of the ARP command, type arp -? at a command prompt.

Verify the Default Gateway

Use the IPCONFIG command to determine the IP address that your computer uses to access your default gateway. To do so, type "ipconfig" (without quotation marks) at a command prompt. Verify that the IP address displayed for your default gateway is correct. If you do not know the correct IP address for your default gateway, contact your network administrator.

When you have verified that you have the correct IP address for your default gateway, use the PING command to verify that you can ping your default gateway's IP address. You should receive a response similar to the following:
   Pinging <###.###.###.###> with 32 bytes of data:
   Reply from <###.###.###.###>: bytes=32 time=77ms TTL=28
   Reply from <###.###.###.###>: bytes=32 time=80ms TTL=28
   Reply from <###.###.###.###>: bytes=32 time=78ms TTL=28
   Reply from <###.###.###.###>: bytes=32 time=79ms TTL=28
				
Where <###.###.###.###> is the IP address of your default gateway.

If your default gateway is not connected to the network or not functioning properly, you may receive a response similar to the following:
   Pinging <###.###.###.###> with 32 bytes of data:
   Request timed out.
   Request timed out.
   Request timed out.
   Request timed out.
				
If you cannot successfully ping your default gateway's IP address, contact your network administrator to verify that your default gateway is connected to the network and functioning properly.

Ping the IP Address of the Other Computer

Try to ping the IP address of the other computer. To do so, type ping <IP address> where <IP address> is the IP address of the other computer. You should receive a response similar to the following:
   Pinging <###.###.###.###> with 32 bytes of data:
   Reply from <###.###.###.###>: bytes=32 time=77ms TTL=28
   Reply from <###.###.###.###>: bytes=32 time=80ms TTL=28
   Reply from <###.###.###.###>: bytes=32 time=78ms TTL=28
   Reply from <###.###.###.###>: bytes=32 time=79ms TTL=28
				
Where <###.###.###.###> is the IP address of the other computer.

If there is an improperly configured router between your computer and the other computer, or if there is a problem with the other computer, you may receive a response similar to the following:
   Pinging <###.###.###.###> with 32 bytes of data:
   Request timed out.
   Request timed out.
   Request timed out.
   Request timed out.
				
If your computer is on a different subnet than the other computer, try to ping the other computer from a computer that is on the same subnet as the other computer. If you cannot ping the other computer from a computer on the same subnet, ensure that the other computer is connected to the network and that you have the correct IP address for the other computer. If you can ping the other computer from a computer on the same subnet, contact your network administrator to resolve any routing problems that may exist on your network.

Verify Persistent Route Table Entries

Any computer using TCP/IP as a network protocol has a route table. The route a network packet takes from one computer using TCP/IP to another computer using TCP/IP is determined by the route table of the computer that sent the network packet.

Your computer's route table is automatically rebuilt each time you restart your computer. You or your network administrator can add persistent (static) entries to your computer's route table. Persistent entries are automatically reinserted in your route table each time your computer's route table is rebuilt.

To view your computer's route table, use the ROUTE command. To do so, type route print at a command prompt. You should receive a response similar to the following:
   Active Routes:

   Network Address   Netmask           Gateway Address  Interface   Metric

   0.0.0.0           0.0.0.0           10.1.1.254       10.1.1.3       1
   10.1.0.0          255.255.0.0       10.1.1.3         10.1.1.3       1
   10.1.1.3          255.255.255.255   127.0.0.1        127.0.0.1      1
   10.255.255.255    255.255.255.255   10.1.1.3         10.1.1.3       1
   127.0.0.1         255.0.0.0         127.0.0.1        127.0.0.1      1
   224.0.0.0         224.0.0.0         10.1.1.3         10.1.1.3       1
   255.255.255.255   255.255.255.255   10.1.1.3         10.1.1.3       1
				
Verify with your network administrator that all persistent entries in your computer's route table are valid.

For more information on routing, route tables, and the ROUTE command, see the following article in the Microsoft Knowledge Base:
140859  (http://support.microsoft.com/kb/140859/EN-US/ ) TCP/IP Routing Basics for Windows NT

Use the TRACERT Command

The TRACERT command reports each router or gateway crossed by a TCP/IP packet on its way to another host. To use the TRACERT command to trace the route between your computer and the other computer, type tracert <IP address> at a command prompt, where <IP address> is the IP address of the other computer. You should receive a response similar to the following:
   Tracing route to <IP address> over a maximum of 30 hops:

     1   <10 ms   <10 ms   <10 ms  <###.###.###.###>
     2    50 ms    50 ms    51 ms  <###.###.###.###>
     3   250 ms    80 ms    50 ms  <###.###.###.###>

   Trace complete.
				
Where each <###.###.###.###> is the IP address of a different router.

If there is a problem with one of the routers that the network packet tries to cross, you may receive a response similar to the following:
   Tracing route to <IP address> over a maximum of 30 hops:

     1   <10 ms   <10 ms   <10 ms  <###.###.###.###>
     2     *        *        *     Request timed out.
     3     *        *        *     Request timed out.
     4     *        *        *     Request timed out.
				
If there is a configuration error on one of the routers between your computer and the other computer, you may receive a response similar to the following:
   Tracing route to <IP address> over a maximum of 30 hops:

     1   <10 ms   <10 ms   <10 ms  <###.###.###.###>
     2    50 ms    50 ms    51 ms  <###.###.###.###>
     3  <###.###.###.###>  reports: Destination net unreachable.
				
You may also receive a response similar to the one above when there is a proxy or a firewall between your computer and the other computer.

If you are not able to obtain a successful response using the TRACERT command to trace the route between your computer and the other computer, contact your network administrator to determine if there is a routing problem between your computer and the other computer.

For more information on the TRACERT command, see the following article in the Microsoft Knowledge Base:
162326  (http://support.microsoft.com/kb/162326/EN-US/ ) Using TRACERT to Troubleshoot TCP/IP Problems in Windows NT

Verify Server Services on the Other Computer

Verify that the appropriate server services are running on the other computer. For example, if you are attempting to use the Telnet tool to connect to the other computer, make sure that the other computer is configured as a Telnet server.

To verify that the appropriate server service is running on the other computer, attempt to connect to the other computer from another computer that is on the same subnet as the other computer. If you cannot connect to the other computer from a computer on the same subnet, contact the network administrator to verify that the server service is configured and functioning properly on the other computer. If you can connect to the other computer from a computer on the same subnet, contact your network administrator to resolve any routing problems that may exist on your network.

Check IP Security on the Server

Port settings for services on the other computer may be different than the port settings you are using to connect. The following chart lists some standard port settings for various protocols:
Port:   Protocol:
-----   ---------
80      HTTP
21      FTP
23      Telnet
70      Gopher
				
Use the Telnet tool to verify that the other computer is configured to permit connections on the same port you are using to connect. To do so, type the following line at a command prompt:
telnet <IP address> <port>
Where <IP address> is the IP address of the other computer and <port> is the port you are attempting to make a connection on. For example, if you are attempting to make an ftp connection to the other computer on port 21, type telnet <IP address> 21.

If you do not receive an error message, the other computer is configured to permit connections on that port. You should be able to make a connection using the appropriate service on that port.

If you receive an error message, the other computer may not be configured to permit connections on that port. Contact the network administrator to obtain a valid port number for the service on the other computer.

Unable to Connect to a Specific Host or NETBios Name

If you are able to connect to the other computer using its IP address, but you are not able to connect to the other computer using its host or NetBIOS name, there may be a name resolution problem. There are many methods that can be used to accomplish name resolution on a network including the following:
  • HOSTS files

  • Domain Name Service (DNS)

  • LMHOSTS files

  • Windows Internet Name Service (WINS)
If you are uncertain which methods are used to accomplish name resolution on your network, contact your network administrator. Follow the procedures in each of the following sections in order. After you finish each procedure, check to see if you can connect to the other computer using its host name or NetBIOS name.

Check the HOSTS File

The HOSTS file is a text file that you can edit with any text editor (such as Notepad). If your network uses HOSTS files for host name resolution and you cannot connect to the other computer using its host name, there may be an invalid entry in your HOSTS file. Search your HOSTS file for the host name of the other computer, verify that there is only one entry per host name, and then verify that the entry for the host name of the other computer is valid.

For more information on the HOSTS file, see the sample HOSTS file in the %SystemRoot%\System32\Drivers\Etc folder.

Check Your Domain Name Service (DNS) Configuration

A Domain Name Service (DNS) server provides host name resolution. If your network uses DNS for host name resolution and you cannot connect to the other computer using its host name, there may be a problem with your computer's DNS configuration or with the DNS server on your network.

To determine if there is a problem with your computer's DNS configuration, follow these steps:
  1. Type ipconfig /all at a command prompt to display the IP address of your DNS server. If the IP address for your DNS server is not displayed, contact your network administrator to obtain the IP address for your DNS server.
  2. Verify that you can communicate with your DNS server by pinging your DNS server's IP address. You should see a reply similar to the following:
          Pinging <###.###.###.###> with 32 bytes of data:
    
          Reply from <###.###.###.###>: bytes=32 time=77ms TTL=28
          Reply from <###.###.###.###>: bytes=32 time=80ms TTL=28
          Reply from <###.###.###.###>: bytes=32 time=78ms TTL=28
          Reply from <###.###.###.###>: bytes=32 time=79ms TTL=28
    						
    Where <###.###.###.###> is the IP address of the DNS server.
If you cannot ping the IP address of your DNS server successfully, contact your network administrator to verify that you have the correct IP address for your DNS server and that your DNS server is connected to the network and functioning properly.

If you can ping the IP address of your DNS server, but cannot resolve the host name of the other computer, your DNS server may not be resolving host names properly. If more than one DNS server is available on your network, configure your computer to use a different DNS server. If another DNS server resolves the host name of the other computer properly or if there is no other DNS server to use, contact your network administrator to correct the problem with the original DNS server.

When you have verified the correct IP address for your DNS server, update your computer's TCP/IP settings. If you are using a dial-up connection to connect to your network, you need to change only the TCP/IP settings in your Dial-Up Networking phone book entry with the correct IP address for your DNS server.

To change or add a valid IP address for your DNS server in your computer's TCP/IP settings, follow these steps:
  1. In Control Panel, double-click Network, and then click the Protocols tab.
  2. Click TCP/IP Protocol to select it, click Properties, and then click the DNS tab.
  3. If you are adding a DNS server, click Add. If you are editing an existing server, click the IP address for the appropriate DNS server, and then click Edit.
  4. Type the correct IP address for the DNS server, and then click OK.
  5. Click OK, and then click OK again. You may need to restart your computer after this step.
To change or add a valid IP address for your DNS server for a Dial-Up Networking phonebook entry, follow these steps:
  1. In Dial-Up Networking, click the appropriate entry in the Phonebook Entry To Dial box.
  2. Click More, and then click Edit Entry And Modem Properties.
  3. Click the Server tab, and then click the TCP/IP Settings button.
  4. Click Specify Name Server Addresses, and then type the correct IP address in the Primary DNS box.

Check the LMHOSTS File

The LMHOSTS file is a text file that you can edit with any text editor (such as Notepad). If your network uses LMHOSTS files for NetBIOS name resolution and you cannot connect to the other computer using its NetBIOS name, there may be an invalid entry in your LMHOSTS file. Search your LMHOSTS file for the NetBIOS name of the other computer, verify that there is only one entry per NetBIOS name, and then verify that the entry for the NetBIOS name of the other computer is correct.

If there are any #INCLUDE entries or any #BEGIN_ALTERNATE to #END_ALTERNATE blocks of lines in your LMHOSTS file, temporarily disable all such lines or blocks of lines by placing the pound (#) character and one space at the beginning of each line to be disabled.

If disabling these lines or blocks of lines resolves the problem, re- enable the lines or blocks of lines one at a time until the problem reoccurs. When you have determined that a specific line or block of lines causes a problem, check the LMHOSTS files that the lines point to.

For more information on the LMHOSTS file, see the Lmhosts.sam sample file located in the %SystemRoot%\System32\Drivers\Etc folder.

Check Your Windows Internet Name Service (WINS) Configuration

A Windows Internet Name Service (WINS) server provides NetBIOS name resolution. If your network uses WINS for NetBIOS name resolution and you cannot connect to the other computer using its NetBIOS name, there may be a problem with your computer's WINS configuration or with the WINS server on your network.

To determine if there is a problem with your computer's WINS configuration, follow these steps:
  1. Type ipconfig /all at a command prompt to display the IP address of your WINS server. If the IP address for your WINS server is not displayed, contact your network administrator to obtain the IP address for your WINS server.
  2. Verify that you can communicate with your WINS server by pinging your WINS server's IP address. You should see a reply similar to the following:
          Pinging <###.###.###.###> with 32 bytes of data:
    
          Reply from <###.###.###.###>: bytes=32 time=77ms TTL=28
          Reply from <###.###.###.###>: bytes=32 time=80ms TTL=28
          Reply from <###.###.###.###>: bytes=32 time=78ms TTL=28
          Reply from <###.###.###.###>: bytes=32 time=79ms TTL=28
    						
    Where <###.###.###.###> is the IP address of the WINS server.
If you cannot ping the IP address of your WINS server, contact your network administrator to verify that you have the correct IP address for your WINS server and that your WINS server is connected to the network and functioning properly.

If you can ping the IP address of your WINS server, but cannot resolve the NetBIOS name of the other computer, your WINS server may not be resolving NetBIOS names properly. If more than one WINS server is available on your network, configure your computer to use a different WINS server. If another WINS server resolves the NetBIOS name of the other computer properly or if there is no other WINS server to use, contact your network administrator to correct the problem with the original WINS server.

When you have verified the correct IP address for your WINS server, update your computer's TCP/IP settings. If you are using a dial-up connection to connect to your network, you need to change only the TCP/IP settings in your Dial-Up Networking phone book entry with the correct IP address for your WINS server.

To change or add a valid IP address for your WINS server in your computer's TCP/IP settings, follow these steps:
  1. In Control Panel, double-click Network, and then click the Protocols tab.
  2. Click TCP/IP Protocol to select it, click Properties, and then click the WINS Address tab.
  3. Type the correct WINS server IP address in the Primary WINS Server box, and then click OK.
  4. Click Close, and then click Yes to restart your computer.
To change or add a valid IP address for your WINS server for a Dial-Up Networking phonebook entry, follow these steps:
  1. In Dial-Up Networking, click the appropriate entry in the Phonebook Entry To Dial box.
  2. Click More, and then click Edit Entry And Modem Properties.
  3. Click the Server tab, and then click the TCP/IP Settings button.
  4. Click Specify Name Server Addresses, and then type the correct IP address in the Primary WINS box.

APPLIES TO
  • Microsoft Windows 2000 Server
  • Microsoft Windows 2000 Advanced Server
  • Microsoft Windows 2000 Professional Edition
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Server 4.0 Standard Edition
  • Microsoft BackOffice Small Business Server 2000 Service Pack 1
  • Microsoft Windows 98 Standard Edition
  • Microsoft Windows Millennium Edition
  • Microsoft Windows 95
Keywords: 
kbhowto kbnetwork KB169790
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