TITLE
    TCP/IP: Network Troubleshooting
Article ID:
Created:
Modified:
30626
7/21/98
7/12/00

TOPIC

    This article tells how to troubleshoot difficulties with TCP/IP connections on a Macintosh computer.


DISCUSSION

    Whenever clients report difficulties accessing IP services, it is good practice to verify IP connectivity at the client and at the server. Even if the client connected successfully in the past, many different things could cause unsuccessful attempts at any time, such as:

      • TCP/IP settings accidentally changed.
      • Routers down.
      • DNS services down.
      • Duplicate IP address appears on the network.
      • Address server (DHCP, BootP) not functioning/not accessible.
      • Corruption is some system files or TCP/IP preference files.
    Ping Utilities
    Testing IP connectivity is often done using utilities that send ping packets. Ping packets are simply small packets from one host to another, that request a response from the recipient. Many ping utilities are available for free download on the Internet. One favorite is MacTCP Watcher. A ping utility is invaluable when troubleshooting IP connections, and the following suggestions are assuming that some ping utility is available.

    If no ping utility is available, then use whatever IP applications are at hand; the simpler the better. Remember that the device you are attempting to contact has to support the service that the client software is trying to reach. You may be able to use a Web browser to test a connection with another Macintosh that has Personal Web Sharing enabled, but you cannot use it to test a connection to your router.

    Here is a suggested troubleshooting path for isolating difficulties with a client trying to connect to the host.

    Ping the host using its Domain name
    If pinging succeeds, then there is no fault with the client or the IP connection between client and host. Your next step would be to check the applications at client and host, and look for faults there (Web server, file server, mail client, and so forth).

    If pinging fails, it could indicate a fault with the routers or DNS servers, not necessarily with the workstation itself. Go on to the next step.

    Ping device on local subnet using IP address
    Pinging a device on your same subnet using its IP address does not require routers or DNS servers to be successful, so it verifies that each device is able to communicate, at least on its own subnet, using TCP/IP.

    Important: be sure that the workstation you are attempting to ping has its IP stack (protocol set) initialized. Most Macintosh computers do not initialize the IP stack until an application that uses IP is opened. On the Macintosh you'll be pinging, open an IP application, such as the Web browser or a ping utility, to initialize the stack before attempting the ping.

    You can determine which other devices are on the same subnet by checking the physical connections (are they connected to the same hub? or same bus?) or by checking their IP addresses (are the network portions of the IP addresses identical? You have to consider the IP address AND the subnet mask to determine this).
      • If pinging a device on the same subnet is successful, you have verified that the computer's IP connections are working, but there still may be some fault with external network devices, such as routers, DNS servers. Continue on with the next step.
      • If pinging a device on the same subnet fails, then you must check carefully the local software and hardware configurations.
    Here are some tips on how to do that:
      • See if any other device on same subnet can ping other devices. This lets you know if the local network is functioning, and if basically the IP addresses have been set up correctly.
      • If server addressing is being used, see if you can test with a manual address, to see if the fault is with communications between the client and address server.
      • Check TCP/IP configuration carefully; compare them with that of a Macintosh that is working properly: verify link being used, IP address, subnet mask, and router information, whether "802.3" is checked (Advanced mode)
    NOTE: Usually 802.3 should be UNchecked; only check it if ALL devices are using 802.3 for TCP/IP communications.

    - Check network connections:
      • make sure they are not loose.
      • swap out network hardware with a working computer.
      • test AppleTalk on the same link (network interface) if possible.
    - If none of the above fixes the issue, try:
      • resetting PRAM.
      • tossing TCP/IP preferences and MacTCP DNR, and reconfiguring.
      • turning off non-MacOS extensions.
      • reinstalling Open Transport, or perform a clean installation of system software.
    Ping router port on same subnet
    The workstation's TCP/IP panel may be configured with a router address. (This should be the router port which is on the same subnet as the workstation itself.) All communications with devices not on the same subnet as the workstation must pass through this router, so if it is not available, your client will not be able to talk to anyone outside the subnet.
      • Using a ping utility, attempt to ping the router port.
      • If pinging the router is successful, go to next step.
      • If pinging the router fails, see the network administrator, or whoever manages the router, and report the issue.

    Ping DNS server
    Whenever a host attempts to make a connection using a domain name, that name must first be resolved to an IP address. The name is sent to the first DNS server listed in the TCP/IP control, and if that server does not respond, it will be sent to the second, the third, and so on. Try pinging the DNS servers listed.
      • If pinging the DNS server succeeds, MacTCP Watcher lets you do a DNS Lookup; see if you can resolve a domain name (try several names in different domains, such as www.apple.com, www.cornell.edu, phc.mpr.org). If resolving names fails, it is possible that the server is down (or the address is not that of a DNS server after all). Verify the information with the network administrator.
      • If pinging the DNS server fails, report it to the network administrator. In the meantime, you can reconfigure your TCP/IP control panel with an address of a working DNS server; check the configurations of other devices on the network who are able to resolve names successfully.

Document Information
Product Area: Communications-Networking
Category: Network Management
Sub Category: General Topics

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