LAN & Network Diagnostics
These notes cover a variety of LAN trouble situations. Hope they help.
Before you start
Before you do anything, before you touch anything else:
1. If you are tired or hungry you will not be at your best. If you are pulling your hair out in chunks you will not make smart decisions. Sometimes the best starting point for diagnosis is to take a break, have a rest or eat something - no matter what the short term pressures.
2. Write down a list of everything that has changed. Chances are high your problem lies with the change or a side-effect of the change.
3. Think it through first - running around doing things with no plan is a recipe for another disaster - once you find the original problem you've then got to go back and fix all the other problems you introduced when doing your imitation of a headless chicken.
4. Be methodical. Start at one place and work slowly out from there. Test each step and MAKE NO ASSUMPTIONS. Check everything.
<rant> If you have got network cables with broken tabs you deserve everything you get. Breathe on those guys and they come loose. Keep a pair of scissors handy and cut the connectors off. Period. </rant>
What you should have done!
Wise after the event advice is never very helpful - but:
1. Know what the NORMAL look of LEDs, flashing light and other indicators are. We typically look at these indicators only when something is NOT working and then assume they are telling us things that they probably are not.
2. Keep a 'gold standard' - something that you know works - could be a laptop, could be a long cable that will stretch everywhere and you KNOW WORKs. Use this to eliminate possible errors.
RJ45 End Connectors
1. Make and test practice cables until you get it right every time - especially before you destroy cables you just spend 2 hours fitting.
2. When cutting the exterior cover of the cable be very careful not to cut the insulation cover of the conductors since this can cause shorts.
3. Expose a maximum of 1 inch of individual conductors when preparing the cable for connection.
4. Line up all the conductors according to the wiring standard you are using.
5. Measure the cable and trim the conductor ends so they are are all the same length and no individual conductor wire is visible outside the plastic cover of the RJ45 connector.
6. Carefully slide the prepared cable into the RJ45 connector making sure the end of the conductors reaches the end of the RJ45 connector.
7. Using the crimp tool make the connection using one firm squeeze operation.
8. Test the cable before fitting if possible.
9. The RJ45 connector is the critical connection always use the highest quality connectors you can afford. The most common cause of connection faults is bad connectors.
1. Create and keep a 'gold standard' i.e. take a single PC/laptop connect it directly to the hub via a cable that is long enough to stretch from every room in the facility to the hub - confirm that the Hub/Switch LINK led works every time and preferably use a PC that has a LINK led on its LAN card.
2. Using this configuration move it to every location when you test the connection. At least you know something works. Plug this configuration into the wall connector under test and if you don't get LINK - its your cable.
3. Test every PC to wall LAN cable before you start by using them to connect to the Hub/Switch locally - you don't want to rip-up your installed cable until you are sure it's at fault.
4. Make no assumptions - test or check everything
5. Buy a cheap magnifying glass - its almost impossible to do a visual check of punch down connections without one
6. Use a 'continuity tester' or a volt-meter to check individual conductors in cables
Networks
'Ping' (actually its full name is 'ICMP Echo request') is a simple command that may be issued from the DOS Command Prompt (Start/Run/cmd/OK). Ping tells you if you can, or cannot, contact an IP address (another PC). Ping sends a small message to another computer which causes the receiver to echo back the same message (the message pings forward and backward). Ping is the simplest and most useful diagnostic tool to become familiar with and well worth a few minutes experimentation. To use Ping;
1. Run a command prompt (sometimes called a 'DOS box') (Start->Programs->Command Prompt (MSDOS) or Start->run enter command -> OK
2. Type:
3. ping xxx.xxx.xxx.xxx
where xxx.xxx.xxx.xxx is the IP address that you want to check followed by ENTER. You can also use a URL with a ping:
ping www.sitename.com
For this format to work the DNS service must be contactable and working.
4. If ping works (you have successfully sent a message to the remote computer and received a response) you should get up to 4 replies of the form:
5. Reply from xxx.xxx.xxx.xxx: bytes=32 time=yyms TTL=zz
Where xxx.xxx.xxx.xxx is the IP address that is responding, yyms is the time (yy) in milliseconds (ms) that the ping took and zz can be used to calculate the number of routers that it passed through on its journey.
6. If the Ping fails you will see a message like:
7. Request timed out (connection or routing error)
8. OR
9. Host unreachable (routing error)
10. OR
11. Network unreachable (routing error)
To diagnose a network problem you just start to verify the connections from a known starting point (your PC) moving progressively further into the network until you find the problem:
1. Always start with your own PC (it regularly fails). Issue a ping to your own PC (get it's address here). If this fails restart your PC and try the failing operation again.
Check with someone else in the office - if you are the only person having the problem you have already isolated it to your PC or its wiring. Now you only have to find it - in all cases it is not a remote network problem.
1. Restart your PC - 90% of all problems disappear with this one act.
2. Check the link LEDs on your PC LAN card (if it has any).
3. Check your cabling.
2. Now check your local network.
Your local PC is OK - someone else has same problem.
Ping the local router (its address is the Default Gateway IP that you get here). If this fails we may have a problem with the local LAN or the router.
Now you have to move from your desk.
Find your router and Hub/Switch check the LED displays. If they are not normal remove and immediately replace the power connection or switch the unit OFF then ON.
Go back to your PC and retry to ping your local router and then repeat the failing operation.
3. Check the remote network.
Your local router is OK - we can reach it and its LEDs are normal.
Issue a tracert command to 64.58.76.176 (www.yahoo.com) NOTE: always use the -d option with this command and you can abandon it using CTRL+C when you see two three consecutive rows of '*'.
Note the hop number of the first failure and contact your service provider.
A tracert (or trace route) command tells you all the routers between your PC and the place you want to trace to (can be either an IP address or a URL e.g. www.smokeyjoe.com )
To run a tracert command:
1. Click start->programs->Command Prompt (or Start->run enter cmd then OK)
2. Enter:
3. tracert www.xxx.xxx.xxx -d' (or www.smokeyjoe.com -d)
Replace the IP address with the one you want or use the URL of the site if you know it.NOTE: The -d in the command line stops a DNS lookup and speeds up the command considerably.
4. tracert outputs the following display:
5. a bbbb cccc dddd ee.ee.ee.ee
Where:
a is the hop number starting from 1
bbbb is the time is milliseconds that the first attempt took to reach the site. Asterisk means it timed out.
cccc is the time is milliseconds that the second attempt took to reach the site. Asterisk means it timed out.
dddd is the time is milliseconds that the third attempt took to reach the site. Asterisk means it timed out.
ee.ee.ee.ee is the IP address of the router at this hop number.
NOTES:
1. If you do NOT add the -d then after the dddd entry you will see a router name first then an IP address in square brackets.
2. You can abandon the command at any time by typing CTRL + C.
There are two methods of doing this - the quick and the long method depending on how much information you want:
The quick method (limited configuration):
1. Load a Command Prompt (a DOS box) (Start->programs->Command Prompt or Start->run enter command then OK)
2. If using windows '95 enter:
3. winipcfg
This will display your IP address, subnet mask and default gateway (local router)
4. If using Windows NT, windows '98, Win2K, XP enter
5. ipconfig /all
this will display your IP address, subnet mask, default gateway (local router) and MAC address
The long method (full configuration)
Windows '95, '98 and NT 4.x, Windows 2K:
1. Click start\settings\control panel.
2. Double click 'Network'
3. Select 'Protocols' tab
4. Select TCP/IP then click 'Properties'
5. Navigate to the relevant tab to find the required information.
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