Quick Network Connection Troubleshooting

Networks are a collection of magical black boxes. From time to time the black boxes may go wrong or the wires interconnecting them may develop faults. Whilst a Tech-contact needn't understand exactly what the black boxes do. Anyone (not just a Tech-contact) can make a useful stab at diagnosing a network fault. If you go through the steps outlined below the likelihood is that you will rapidly be able to diagnose the problem or at least to describe the symptoms accurately to the Help Desk.

Networks transmit electronic (sometimes light) packets of information. If a network (or part of a network) is not working, then it's because these packets are not getting through. The two most common causes of packets not getting to their destination are:

A physical break in connection (a wire is unplugged or broken)
A magic box is not working properly

However, many users' reports of "network" problems turn out to be something else entirely so first establish the following:

Configuration of the local system?
Pay particular attention to the setting of the "netmask" and the IP address. Check that "routing" information is available, either through the explicit configuration of a default route (most usual scheme for personal computers) or through an active routing daemon (most usual scheme for workstations). If you are using a routing daemon, additionally check the setting of the IP broadcast address. If any of these is set incorrectly you can expect problems using the network.

Incorrect settings of any of the above are amongst the most common causes of network problems of newly connected, or configured, devices.

Valid service?
It's obvious but check that the network service that is reported as causing a problem is indeed a valid service. For example, some IP systems are registered in the Domain Name Server (DNS) without addressing information, so it is not legitimate to attempt to "connect" to them.

Restricted access?
Some network systems restrict access to some of their services to "correctly" DNS registered devices (for example BIDS - Bath Information and Data Services). Check that the system is correctly DNS registered if you suspect that this may be the cause of the problem.

PING!

Let's get pinging! The principle weapon for network troubleshooting is "ping" - a sort of network echo sounder (ping stands for Packet InterNetGroper). Ping is available for many computer platforms. Armed with Ping you can check the connectivity of your network very quickly. Ping sends a packet from one device, attempts to "bounce" it off another, and "listens" for the reply. If you get a prompt reply then the bit of network between the pinging device and the "ping" device is okay. The best strategy when troubleshooting is to start pinging devices that are near to you and then to try to ping devices that are progressively more remote. If you fail to ping a device then you've isolated the problem.

Some points about the use of ping
You do need to ensure that the local device you're attempting to ping is a ping responder. (Check this now whilst you're network is working normally.) Not all subnetworks have pingable bridges.

The way in which ping may fail can sometimes give a clue as to what's wrong. The kind of failure messages you'll see are:

No answer from <destination>

No reply from <destination>

This is the most usual failure you'll see and indicates that your local system at least thinks it knows how to get to the destination.

ICMP Host Unreachable from gateway

This is an interesting one and indicates that while your system knows that it needs to get at the destination via a gateway, that the gateway itself doesn't know how to route packets on. It may be because you have set the local gateway address incorrectly and does not necessarily indicate a network problem.

<destination> is unreachable

This indicates that your local system does not know how to get to the destination. Often caused by not having any routing information available.

So, by acting rationally and systematically, we have seen how to use ping to isolate a network fault. Of course not all network problems can be resolved by this relatively simple-minded approach. That said, if you go through these steps BEFORE reporting the problem to us, you will save time and trouble for all concerned!

If you still need help, please contact your local computer vendor for hardware issue or system integrator vendor for possible network configuration support and assistance.

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