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Troubleshooting an IP-based Network Connection

Article Number: H006
Article Type: HowTo
Modified: November 23, 1998

SUMMARY

This article describes a problem-solving methodology to diagnose dial or leased-line connections where a logical connection cannot be established between the end devices.

In an IP-based network, a logical path is required between all end devices in both directions in order for a station on one network to communicate to a station on another network. This logical path is nominally composed of both a physical path (the link) and a suitable collection of routes in the routing tables of each device along the physical path. This will include the end stations themselves.

This article steps you through the process of determining why a particular network station is unable to reach another over a routed network connection.

MORE INFORMATION

Example Network Setup

The remainder of this article uses the setup depicted in the following diagram:

To locate the source of the problem, a variety of tools are readily available. Please refer to the REFERENCE section for a list of those tools included with your operating system (usually undocumented by the supplier).

The process to determine the reachability of a remote workstation documented in this procedure relies on the use of PING. PING is a simple utility that is available on most all IP-based workstations, servers and devices, and is an effective tool for establishing the reachability of a station.

Step 1 - PING Yourself First!

A very simple, yet often overlooked step is to PING yourself first. Doing so validates the configuration of your PC (Computer A), and if unsuccessful, there is no need to proceed any further until this can be corrected.

	ping 192.168.1.2

If a response is received, proceed to step 2.

If this fails, check that all network card parameters have been correctly configured, and that the card is correct connected to the LAN.

Step 2 - PING the Local Router

If we have successfully passed step 1, we can now move downstream one logical hop--this takes us to Router A.

By performing a PING to this device, we will verify that the LAN connectivity and the router is properly configured on the same LAN.

	ping 192.168.1.254

If a response is received, proceed to step 3.

If this step fails, check the following:

• Is Router A powered up?
• Are both Computer A and Router A connected to the same LAN segment?
• Is the router's IP Address part of the same logical IP network as the PC?

Step 3 - PING the Remote Router

The next step involves trying to reach Router B in our example. Successfully passing this step validates a great many things, but if failure occurs, this part of the example may also be the most complicated to solve.

	ping 192.168.2.254

If a reply is received, you have just verified that both Router A and Router B are properly connected, have valid routes to each other, that Computer A has a proper route to Network B, and that all units involved in this step are communicating. You may move to the next step.

If this step fails, check the following:

• Is Router B powered up?
• Has both Router A and Router B detected the presence of the Link?
• Is there either a specific or a default route on Computer A for network 192.168.2.0?
• Is there either a specific or a default route on Router A for network 192.168.2.0?
• Is there either a specific or a default route on Router B for network 192.168.1.0?
• Is Router B configured with the proper IP Address?

Step 4 - PING Computer B

This last step verifies that Computer B is now properly configured.

	ping 192.168.2.1

If we get a reply, we have confirmed end-to-end connectivity between these two networks.

If a reply is not received start again from Step 1, but now work in reverse from Computer B back through to Computer A. Start by going to Computer B and try to ping itself, ping Router B, and so on until you have verified the cause of the problem.

An Example Router Table for Computer A

Network Address	Netmask		Gateway Address	Interface

0.0.0.0         0.0.0.0		192.168.1.254	192.168.1.2
127.0.0.0       255.0.0.0	127.0.0.1	127.0.0.1
192.168.1.0    	255.255.255.0	192.168.1.2	192.168.1.2
192.168.1.2  	255.255.255.255 127.0.0.1	127.0.0.1
192.168.1.255  	255.255.255.255	192.168.1.2	192.168.1.2
224.0.0.0       224.0.0.0	192.168.1.2	192.168.1.2
255.255.255.255 255.255.255.255	192.168.1.2	192.168.1.2

Note that in this example, Computer A has a default route (represented by the 0.0.0.0 entry) pointing to Router A.

CONSIDERATIONS

The steps outlined above represent a simple view of an IP-based network that may or may not accurately reflect your network. Other issues such as security, intermediate router hops, variations in LAN media, link protocols, and even incompatibilities of application-layer entities may affect the outcome of your tests. In validating your network, you will have to identify how these other components may impact or modify your results.

REFERENCES

Utilities supplied with most operating systems are only installed when the TCP/IP stack is loaded. On Windows machines (including Windows 98) they are available at the MSDOS prompt. Typing the command will generally show you the options available.

• Netstat - Provides a variety of useful information
• Ping - IP utility to show the presence and distance in hops to a target
• Route - Most useful to display the PC's routing table, and can also allow routes to be added and removed.
• Tracert - Traces the delay through each of a number of hops to get to a destination. Very useful in locating the slower hops when there is poor response from a distant machine
• ARP - Displays the current relationship between MAC addresses and IP addresses, and allows the table to be manipulated
• Telnet - Used for remote control of routers, as well as logging in to remote computers

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Copyright © 1998 Develcon Electronics Ltd.  All Rights Reserved.