The Process of Elimination and Divide
and Conquer techniques are the most successful methods for networking
troubleshooting. The following scenarios explain these
techniques.
The Process of elimination technique
will be applied to the following problem:
A user on your network calls the help
desk to report that their computer can no longer get to the Internet.
The help desk fills out the error report form and forwards it to you,
the network support department.
You call and talk to the user and they
tell you that they have done nothing differently than they have always
done to get to the Internet. You check the hardware logs for the
network and find out that the user's computer was upgraded last night.
Your first solution is that the computer's network drivers must be
incorrectly configured. You go to the machine and check the network
configuration information on the computer. It seems to be correct, so
you Ping the server on that subnet. It doesn't connect. 
The next solution is to check to see if
the workstation cable is plugged in. You check both ends of the cable
and then try Pinging the server again.
Next you Ping 127.0.0.1, the loopback
address for the computer. The Ping is successful, so that eliminates a
possible problem between the computer, driver configuration, and the
NIC card. 
You decide then that there might be a
problem with the server for this network segment. There is another
networked computer at the next desk, so you Ping the server's address
and the result is successful. This eliminates the server, the
backbone, and the server's connection to the backbone as the problem. 
You then go to the IDF and switch the
port for the workstation, go back to the workstation and try to Ping
the server again. The solution still does not work. This narrows your
search down to the horizontal cabling or the workstation patch cable.
You go back to the IDF, put the cable back in the original switch
port, get a new workstation patch cable and return to the workstation.

Replace the workstation cable, and try
to Ping the server again. This time it is successful, so you have
fixed the problem.

The last step is to document the
problem solution on the error report form and return it to the help
desk so it can be logged as completed.
The Divide and Conquer technique will
be applied to this problem:
You are combining two networks that
work fine when not connected, but when they are joined the entire
combined network fails. Refer to the diagram for reference. 
The first step would be to divide the
network back into two separate networks and verify that the two still
operate correctly when separated. If this is true then remove all of
the subnet connections for one of the connecting routers and reconnect
it to the other working network. Verify that it is still working
correctly. 
If the network is still functioning add
each of that router's subnetworks back into the router until the
overall system fails. Remove the last subnet that was added and see if
the whole network returns to normal operation. 
If the network is again functioning
normally, remove the hosts from the network segment and replace them
one at a time, again checking to see when the network fails. When you
find the offending device, remove it and verify that the network
returns to normal. 
If the network is still functioning
normally, you have isolated the faulty piece of equipment. It is now
possible to troubleshoot this individual piece of equipment to find
out why it was causing the entire network to crash. If nothing proves
to be wrong with this device upon analysis, it may be that this device
in conjunction with another device on the opposite network is causing
the problem. To find the other end of the problem you would have to
repeat the process that was used above. This is that process: 
First reconnect the host that caused
the network to fail. Then disconnect all of the subnetworks from the
other router. Check that the network has returned to operating status.

If the network is functioning again,
add each of that router's subnetworks back into the router until the
overall system fails. Remove the last subnet that was added before the
failure and see if the whole network returns to normal operation. 
If the network is again functioning
normally, remove the hosts from the network segment and replace them
one at a time, again checking to see when the network fails. When you
find the offending device, remove it and verify that the network
returns to normal. 
If the network is still functioning
normally, you have isolated the other faulty piece of equipment. It is
now possible to troubleshoot this individual piece of equipment to
find out why it was causing the entire network to crash. If nothing
proves to be wrong with this device upon analysis, compare the two
hosts and find the reason for their conflict. By resolving this
conflict, you will be able to reconnect both stations into the network
and it will still function normally. 
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