tcp-ip-troubleshooting

Important If the remote computer that you are pinging is across a high-delay link such as a satellite link, response may take longer. You can use the -w (wait) parameter to specify a longer timeout period than the default timeout of four seconds.

Method 3: Use the PathPing tool to verify a route

The PathPing tool detects packet loss over multiple-hop paths. Run a PathPing analysis to a remote host to verify that the routers on the way to the destination are operating correctly. To do this, type the following command:

pathping IP address of remote host

Method 4: Use the Arp tool to clear the ARP cache

If you can ping both the loopback address (127.0.0.1) and your IP address but you cannot ping any other IP addresses, use the Arp tool to clear out the Address Resolution Protocol (ARP) cache. To view the cache entries, type any one of the following commands:

arp -a

arp -g

To delete the entries, type the following command:

arp -d IP address

To flush the ARP cache, type the following command:

netsh interface ip delete arpcache

Method 5: Verify the default gateway

The gateway address must be on the same network as the local host. Otherwise, messages from the host computer cannot be forwarded outside the local network. If the gateway address is on the same network as the local host, make sure that the default gateway address is correct. Make sure that the default gateway is a router, not just a host. And make sure that the router is enabled to forward IP datagrams.

Method 6: Use the Tracert tool or the Route tool to verify communications

If the default gateway responds correctly, ping a remote host to make sure that network-to-network communications are working correctly. If communications are not working correctly, use the Tracert tool to trace the path of the destination. For IP routers that are Microsoft Windows 2000-based or Microsoft Windows NT 4.0-based computers, use the Route tool or the Routing and Remote Access snap-in to view the IP routing table. For other IP routers, use the vendor-designated appropriate tool or facility to examine the IP routing table.

Most frequently, you receive the following four error messages when you use Ping during troubleshooting:

TTL Expired in Transit

This error message means that the number of required hops exceeds the Time to Live (TTL). To increase TTL, by use the ping -i command. A routing loop may exist. Use the Tracert command to determine whether misconfigured routers have caused a routing loop.

Destination Host Unreachable

This error message means that no local or remote route exists for a destination host at the sending host or at a router. Troubleshoot the local host or the router's routing table.

Request Timed Out

This error message means that the Echo Reply messages were not received in the designated timeout period. By default, the designated timeout period is four seconds. Use the ping -w command to increase the timeout.

Ping request could not find host

This error message means that the destination host name cannot be resolved. Verify the name and the availability of DNS or WINS servers.

Method 7: Verify Internet Protocol security (Ipsec)

IPSec can improve security on a network, but changing network configurations or troubleshooting problems more difficult. Sometimes, IPSec policies require secured communication on a Windows XP Professional-based computer. These requirements can make it difficult to connect to a remote host. If IPSec is implemented locally, you can turn off the IPSEC Services service in the Services snap-in.

If the difficulties end when you stop the IPSec services, IPSec policies are either blocking the traffic or requiring security for the traffic. Ask the security administrator to modify the IPSec policy.

Method 8: Verify packet filtering

Because of mistakes in packet filtering, address resolution or connectivity may not work. To determine whether packet filtering is the source of a network problem, turn off TCP/IP packet filtering. To do this, follow these steps: To ping an address, use its DNS name, its NetBIOS computer name, or its IP address. If the ping succeeds, the packet filtering options may be misconfigured or too restrictive. For example, the filtering can allow the computer to act as a Web server, but, to do this, the filtering may turn off tools such as remote administration. To restore a wider range of permissible filtering options, change the permitted values for the TCP port, the UDP port, and the IP protocol.

Method 9: Verify the connection to a specific server

To determine the cause of connectivity problems when you are trying to connect to a specific server through NetBIOS-based connections, use the nbtstat -n command on the server to determine what name the server registered on the network.

The nbtstat -n output command lists several names that the computer has registered. The list will include a name that looks similar to the computer's name that is configured on the Computer Name tab under System in Control Panel. If not, try one of the other unique names that the nbtstat command displays.

The Nbtstat tool can also display the cached entries for remote computers from #PRE entries in the Lmhosts file or from recently resolved names. If the name that the remote computers are using for the server is the same, and the other computers are on a remote subnet, make sure that the other computers have the computer's name-to-address mapping in their Lmhosts files or WINS servers.

Method 10: Verify remote connections

To determine why a TCP/IP connection to a remote computer stops responding, use the netstat -a command to show the status of all activity for TCP and UDP ports on the local computer.

Typically, a good TCP connection shows 0 bytes in the Sent and Received queues. If data is blocked in either queue or the state of the queues is irregular, the connection may be faulty. If data is not blocked, and the state of the queues is typical, you may be experiencing network or program delay.

Method 11: Use the Route tool to examine the routing table

For two hosts to exchange IP datagrams, both hosts must have a route to each other, or they must use default gateways that have a route. To view the routing table on a Windows XP-based host, type the following command:

route print

Method 12: Use the Tracert tool to examine paths

Tracert sends ICMP Echo Request messages that have incrementally higher values in the IP header TTL field to determine the path from one host to another through a network. Then Tracert analyzes the ICMP messages that are returned. With Tracert, you can track the path from router to router for up to 30 hops. If a router has failed, or the packet is routed into a loop, Tracert reveals the problem. After you locate the problem router, you can contact the router administrator if the router is offsite, or you can restore the router to fully functional status if the router is under your control.

Method 13: Troubleshoot gateways

If you receive the following error message during configuration, determine whether the default gateway is located on the same logical network as the computer's network adapter:

Your default gateway does not belong to one of the configured interfaces

Compare the network ID part of the default gateway IP address with the network IDs of the computer's network adapters. Specifically, verify that the bitwise logical AND of the IP address and the subnet mask equals the bitwise logical AND of the default gateway and the subnet mask.

For example, a computer that has a single network adapter that is configured with an IP address of 172.16.27.139 and a subnet mask of 255.255.0.0 must use a default gateway of the form 172.16.y.z. The network ID for this IP interface is 172.16.0.0.

Additional resources

The following resources contain additional information about how to troubleshoot Microsoft TCP/IP:

See the "Configuring TCP/IP" topic in the documentation for the Microsoft Windows XP Professional Resource Kit.

See "Introduction to TCP/IP" in the TCP/IP Core Networking Guide of the Microsoft Windows 2000 Server Resource Kit for general information about the TCP/IP protocol suite.

See "Unicast Routing Overview" in the Internetworking Guide of the Microsoft Windows 2000 Server Resource Kit for more information about routing principles.

See "TCP/IP Troubleshooting" in the TCP/IP Core Networking Guide of the Microsoft Windows 2000 Server Resource Kit for more information about IP packet filtering.