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Any help diagnosing a network failure?

  1. Aug 3, 2012 #1
    Something happened last night and one or more things failed in the home network.

    For years it has been three computers running XP with fixed IP addressess hard wired to an SMC 7400VBR router hard wired to a Motorola SB5120 Surfboard cable modem. Nothing wireless. Cyberpower UPS and power filter to keep power clean. No power failures last night or it would have warned me. Computers turned off at night to keep them cooler, router and cable modem left on all the time.

    This morning the computers came up and the network didn't. I've seen that every few years, usually after a power failure they don't initialize cleanly. Pull power on cable modem and router, wait, restore power. Nope. Restart a computer. Nope. Check another computer. Nope. Call the cable company. In a few minutes they decide it is a dead cable modem and I should lease a new one from them or go buy one.

    I don't have a substitute modem and can't find one quickly. Maybe this is a router problem. I eliminate the router and wire one computer directly to the cable modem, change IP address to match and I have connectivity! That tends to tell me it is not a dead cable modem. I try to duplicate this by substituting a second computer. Nope. I try to duplicate this by substituting a third computer. Nope. Back to the first one. Connected. Try swapping new cables. Nope. Don't know why only one works.

    But maybe the first issue was a dead router. Check that. Every computer can see the router admin web page when using fixed IP. So it isn't a completely dead router. I do more checking. The first computer works with DHCP and DNS to the cable modem and fixed IP addresses seem to cause problems. Ah, maybe it is a fixed IP address problem. But the second can see the web page of the cable modem with fixed IP address and can't get DHCP or DNS working or get out to the net. Likewise the third.

    So. Why the difference between computers? Why does one get to the net with auto DHCP and DNS but the other two can't? Why would I be having trouble with fixed IP addresses, the five addresses just aren't that complicated? It seems like I am missing some clue so all this would make sense.

    Any ideas what I should try next?

  2. jcsd
  3. Aug 3, 2012 #2

    I assume some providers do the same thing some once did. They would connect a PC to the cable modem and the MAC from the PC was used as validation of an authorized system using the cable modem vs. someone just buying a cable modem and plugging in. When I added a router, they had me spoof the MAC of the PC to the router so the router would be seen as an authorized connection and the only one the ISP would see or care about. If I changed NIC or dumped the old system, RR would set me up to use the new system MAC. Don't know if it's still done, but checking with your ISP should tell you. Are you always starting the same PC first so the modem sees the same MAC first? I wonder if your system could be spoofing the MAC of the wrong PC. Since it works with one PC only, seems possible.
  4. Aug 3, 2012 #3
    I made a little progress, from looking for clues in dozens of web pages with people asking questions about cable modems and seeing a surprising number of answers that are just simply wrong.

    My current superstition is that I have to cut off power to the cable modem, wait, and reapply power before any single device is directly connected to the cable modem, which then does a DHCP, etc, etc. Either fixed IP address or dynamic for DNS makes no difference. So now each of the computers have successfully individually connected to the net through wire to the cable modem using DHCP.

    If all this is true then that eliminates the puzzle about why one computer would connect just fine and the others, set up identically would not, despite trying this in many many different configurations. If possible, I just need to figure out what it was about that one that made it consistently succeed while the others consistently failed, but that may be too much to ask for.

    The cheap substitute Airlink 101 router does connect all the computers to the cable modem to the net when I cycle the power on the cable modem prior to connecting. Now I have to diagnose the SMC router and if it has failed then I get a new one. But this is great progress.
    Last edited: Aug 3, 2012
  5. Aug 4, 2012 #4
    Better information would also include other key infdicators. For example, lights at both ends of each Ethernet cable report valuable information. Typically a light on the computer's Internet connection. And the router's front panel lights. What are those lights doing especially when a connection does not exist.

    Also what are those same lights for the connection from router to modem.

    Does any one computer see another via the router? Does it do this with modem connected and disconnected? IOW what does PING report with every configuration test.

    Also what exactly is the IP address of each computer when it connects or when it does not. IOW from a Command Prompt, enter IPCONFIG /ALL . You will need those numbers to also properly use PING. Don't starve your help of what is most important - those numbers.

    Ignore DNS. Your only concern should be static IP address and DHCP assigned addresses. Not only that they do not work. But the actual IP address and mask at the router and computer with each test using IPCONFIG, et al.

    Are all filters and firewalls turned off when testing?

    I did not see it. Is MAC spoofing off?

    Make a 'null modem' ethernet cable. That means two computers connect directly to each other. Using static IP addresses, do every two computers talk to each other? And what do those Ethernet lights report?

    Among the many suggestions, you first concern is what computers can talk to what via the 'null modem' ethernet cable AND via the router. Until all that hardware is known, completely ignore the modem.
  6. Aug 7, 2012 #5
    On the two computers that can not connect to the "internet" try the following

    Open command prompt (Windows +R, type: "cmd" press enter/ok)
    Type in as follows

    ipconfig /flushdns

    Press enter and restart the PC

    This clears the DNS resolver Cache, this fixes problems with say one pc working fine but the others dont, fixed IP addresses sometimes have this issue due to having to manually set DNS server settings.

    if your router is using a prefix you should have no problems with a subnet of if your router uses prefix but you only use,, for your pc's and for the modem, just make sure they are using instead of the default
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