Troubleshooting Ethernet network connectivity issues

Network connectivity problems:

  • slow connections
  • dropped connections
  • lost or no connection

Possible causes:

  • local, or network configuration errors
  • incorrectly configured DNS settings
  • inefficiently configured routing tables, or router faults
  • inferior, damaged, or disconnected cabling
  • overloaded servers
  • network congestion
  • server, or power failures

Troubleshooting steps

  1. To begin troubleshooting, check the following top issues. If your issue is listed, select the link, otherwise proceed to step 2.

  2. Verify the presence of a physical connection by ensuring that link LED illuminates when the network cable is connected. Where the network cable plugs into the network card, there are usually 1 or 2 LED indicators. One should be green (either solid or blinking):

    link leds

    If the link LED fails to light, it indicates that no physical connection exists to the network. There may be a problem with either the network cable or the network itself:
    • incompatible, or damaged cables
    • loose connection, or unplugged cables
    • loss of power to a network switch or router

      1. Verify that the RJ45 data cable is firmly and properly connected; and is not cut, frayed or damaged.
      2. Test the connection on a different port of the hub, switch or router. The port may be defective.
      3. Check the other end of the cable. If it is plugged directly into a hub or switch, you should also see an LED indicator that there is a connection. If one or both ends does not have this indicator, try a different cable.
      4. Verify you are using the correct type of cabling for your network application. There are several types of Ethernet cabling, verify specs and lengths:

        Ethernet Standards
        IEEE Specs
        Bus Topology
        Segment Length
        Medium Support
        10BASE5 802.3 10 Mbps Bus 500 meters 50 Ohm thick coax
        10BASE2 802.3a 10 Mbps Bus 185 meters 50 Ohm thin coax
        10BASE-T 802.3i 10 Mbps Star 100 meters 100 Ohm, 2-pair Cat3 UTP
        100BASE-T 802.3u 100 Mbps Star 100 meters 100 Ohm, 2-pair Cat5 UTP
        100BASE-FX n/a 100 Mbps Star 400 meters Single-mode or multimode fiber
        1000BASE-T 802.3ab 1000 Mbps Star 100 meters 100 Ohm, 4-pair Cat5 UTP
        1000BASESX 802.3z 1000 Mbps Star 500 meters Multimode fiber
        1000BASELX 802.3z 1000 Mbps Star 3000 meters Single-mode fiber

      5. Verify that the network adapter is properly seated and is Enabled in BIOS. If the network interface card (NIC) is a PCI or ISA card, reseat the adapter firmly, making sure that the card is fully seated in the slot. If using an onboard or built-in NIC, boot into the BIOS program (F1 on IBM logo screen) and make sure the Ethernet is Enabled. Check your system's Hardware Maintenance Manual (HMM) for details.

      6. Verify that the NIC is compatible with the system and other components in the system. Check the IBM ServerProven Compatibility Web site for tested and certified options.

      7. Make sure all PCI slots are enabled and that the orange power tabs are completely closed on Hot-swap PCI models. If these tabs are not closed, the slot will receive no power and will show "Opn HP pow" in the PCI slot information menu of BIOS (under Advanced Setup).

  3. Test if you can connect to ANY network host. The PING command can be used to check for a basic network connection to an IP address or hostname.

    • If you are unable to connect to ANY network hosts, the problem most likely is with the configuration of your local machine. The local machine may be configured with an incorrect local IP address, incorrect Netmask settings, or incorrect setup within the operating system.
    • If you are able to connect to the local network only, the problem is likely due to incorrectly configured Default Gateway settings.
    • If you are able to connect to the internal (Intranet) network, but not the outside Internet, the problem is likely due to DNS or firewall settings.
    • If you can PING an IP address, but not the corresponding hostname; it indicates a name resolution or DNS Server configuration problem.

  4. Verify that the network adapter is being seen by the operating system. For Microsoft Windows, check Device Manager and look for an icon that appears as Network Adapters and see if the network card is installed:

    1. Right-click the My Computer icon and select Properties.
    2. Click the Hardware tab and press the Device Manager button. The network card should appear under Network Adapters.

      Note: An exclamation mark over a yellow background is an indication that the card is experiencing a conflict.

  5. Check the operating system for resource conflicts between the NIC and some other system component. A NIC should not share an IRQ with SCSI or RAID controllers if possible. These settings can be hard-set within most server BIOS programs.

    1. Press F1 at bootup to enter the Setup/Configuration Utility.
    2. Look under Advanced Setup / PCI Slot/Device Information (or PCI Bus Control) and select the slot or device to be changed.
    3. Disable any unused devices to free up resources (serial ports, parallel ports, and so forth).

  6. Check the network settings within the operating system. If the latest driver is loaded in the operating system and there are no resource conflicts with other devices, check the network settings. Are all protocols and network hardware setup correctly (cables, hubs, switches, etc.)? Are all IP addresses unique or is DHCP setup correctly to automatically obtain a TCP/IP address?

  7. Verify that TCP/IP (or other appropriate protocol) is properly installed. To check this under Microsoft Windows:

    1. Right-click the My Network Places icon and select Properties. The network card should be present as Local Area Connection.
    2. Right-click the appropriate Local Area Connection and select Properties. In the Properties window, make sure that the Internet Protocol (TCP/IP) is installed.
    3. If TCP/IP is not installed, click Install, click Protocol, and then click TCP/IP.
    4. Once installed make sure that Obtain an IP address automatically, and Obtain DNS server address automatically are checked (if using DHCP). To verify this, highlight the TCP/IP protocol, and select Properties. Note: Only use a Static IP Address if your network/Service Provider requires one.

  8. Verify the TCP/IP address. To do this under Microsoft Windows:

    1. Click Start, select Programs, select Accessories, and then click Command Prompt.
    2. Type the command, ipconfig. This will list the IP address for the local machine.
    3. If this returns a 169.x.x.x or address, type the following commands, ipconfig /release and then ipconfig /renew. You should then receive a TCP/IP address appropriate to your network, along with a Default Gateway address.
    4. Try to communicate with it by typing the command ping x.x.x.x (where x.x.x.x is the IP address of the Default Gateway).
    5. If this does not give a reply or you did not receive a TCP/IP address, go to the next step.

  9. Ping the loopback address. To do this under Microsoft Windows:

    1. Click Start, select Programs, select Accessories, and then click Command Prompt.
    2. Type the command, ping This will send a message to the internal network stack on the machine. You should see a response like this:

      Pinging with 32 bytes of data:

      Reply from bytes=32 time<10ms TTL=128
      Reply from bytes=32 time<10ms TTL=128
      Reply from bytes=32 time<10ms TTL=128
      Reply from bytes=32 time<10ms TTL=128

      Ping statistics for
      Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
      Approximate round trip times in milliseconds:
      Minimum = 0ms, Maximum = 0ms, Average = 0ms

  10. Check if you can PING the system's IP address. To troubleshoot the IBM hardware, connect the system directly to another system or a client using a crossover cable or dumb-hub. Setup TCP/IP using two consecutive addresses (e.g. and and use the default subnet masks ( From a command prompt, try to PING the client IP address. If the system gets a response, the IBM hardware is fine. If unable to PING using the above setup, and you are using ethernet, are all cables CAT-5 and hubs 10/100? If not, swap the cable and/or hub. Make sure to use a straight-through cable with a hub and a crossover cable if going directly from server to client. If using a known good crossover cable or hub, try uninstalling and reinstalling the network driver and protocols. Contact your network administrator for more help.

  11. Verify that the most recent driver for the adapter is installed. If the NIC is showing up in the system BIOS or during POST, but it's not recognized by the operating system, make sure the latest driver is loaded. Some native operating system drivers have known performance issues with IBM NICs. Please check the IBM Systems Support Web site for the latest device drivers for IBM servers, IntelliStation workstations, and network interface card options.

    Note: If you are using a card from a different manufacturer, please check the card's documentation for the proper Web site to visit

  12. Run diagnostics on the Ethernet card. Click here to find and download the latest diagnostics for your system.

    Note: If using another manufacturer's product, then consult its documentation on how to run the diagnostics for the Ethernet card.

  13. Install latest Service Packs for Operating System or Network Client. It may be necessary to install Service Packs for either the operating system or for any additional network clients that may be installed. Contact the software vendor for these updates which are usually free downloads.

  14. If these steps have not solved your problem:
    Refer to your system's Hardware Maintenance Manual, or refer to "Need more help?"

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Document id:  MIGR-40284
Last modified:  2011-04-07
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