Microsoft DHCP bugs make Windows lose networking

Scott spanbauer By Scott Spanbauer

Numerous perplexed Windows users have discovered that attempting to connect their PCs (especially Vista) to their existing networks or Wi-Fi hotspots results in flaky or nonexistent connections.

One reason: a change by Microsoft in Vista’s Dynamic Host Control Protocol (DHCP) is causing conflicts with some networking hardware, which can require a Registry edit to fix.

The many reports of Vista networking snafus range from the gravest of symptoms — no Internet connectivity at all — to occasional connection drops:
  • No-Fi when in power-saving mode. Microsoft acknowledged last year that wireless connections on portable computers running Windows Vista would slow down or disconnect completely when battery management kicks in.

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    The culprit is that, unlike Windows XP, Vista assumes that all wireless routers correctly implement Wi-Fi’s power-save protocol. Unfortunately, many access points don’t support this spec. The solution? Plug your laptop into an AC outlet or modify the notebook’s power-saving plan, as described in Knowledge Base article 928152.

  • Vista insists on the “broadcast flag.” The same skewed reasoning led the wizards of Redmond to another infuriating decision, which Microsoft only belatedly explained. You bring home your new Vista computer, or you upgrade your XP system to Vista, only to discover that the machine won’t connect to your local network or the Internet.

    You try everything to fix the problem. You waste hours — days, even — tweaking settings, plugging and unplugging, resetting, rebooting, and rehashing, but to no avail.

    The problem? Windows Vista assumes that your router’s DHCP server — the one that hands out dynamic IP addresses to computers and other devices on the network — supports the DHCP broadcast flag. Again, many routers don’t support this flag.

    The solution requires a Registry edit to toggle off Vista’s broadcast-flag expectations. Refer to the Resolution section of KB article 928233 for step-by-step instructions.

  • Two network adapters spell trouble. Yet another kind of network malfunction afflicts PCs running Vista or Windows Server 2008 that have more than one network adapter installed. The multiple adapters befuddle the Network Location Awareness service in those OSes. This causes the service to disable Internet access to both adapters and label them as Local only.

    KB article 947041 explains the problem but provides no solution. The only cure at this time may be to disable one of the network adapters. Thanks, Microsoft.
How to troubleshoot XP and Vista network woes

Network-connection problems are infuriating. Finding their source requires a step-by-step approach. Before editing your Registry for the umpteenth time or tossing your router into the trash, run through this network-troubleshooting checklist:
  • Temporarily disable your software firewall. It sounds dumb, but often it’s your firewall that’s blocking your network connection. Even if the firewall has worked flawlessly for months, a small configuration change or automatic update could have caused a problem.

    At least twice this year, Windows XP users of Check Point Software’s ZoneAlarm personal firewall have lost their ability to connect to the Internet due to a Windows update. Windows Secrets contribtuing editor Susan Bradley described this problem in her Oct. 16 Patch Watch column (paid content).

    This alone is not a good reason to stop updating, though. It’s true that patches can introduce problems with firewalls, but subsequent fixes that remedy the issue will often appear within 24 hours.

  • Check the physical connection. Make sure the router, modem, and other network devices are plugged in and powered on. Are the network cables between PC and router still connected firmly? With a device’s power switch off, it doesn’t hurt to unplug the component and then plug it back in again to make sure the contact is solid. If weak power-cable connections are ruled out, simply powering the devices off and back on can sometimes be all the resetting your network link needs.

  • Renew your connection. Changes elsewhere on the network can sometimes knock out your connection. To reconnect quickly, click Start, Run in XP (or press the Windows key in Vista), type ipconfig /renew, and press Enter.

  • Update your firmware and drivers. Makers of routers and network adapters may be caught unawares by patches to operating systems (such as the ones in Vista noted above). But the vendors often issue firmware or driver updates that fix the problems. Check the support pages of your router and adapter manufacturers’ sites for downloadable updates.

  • Return to default settings. Often, we are our own worst enemies as we poke around the configuration settings of our routers and network connections. You may not remember that you turned on your router’s MAC filtering, but doing so could have blocked all of your devices from connecting, just the same.

    In general, it’s best to change settings one at a time and observe the results of the change before making any other alterations to your system. If you don’t see an obvious way to return your hardware and software to their default settings, you may have to uninstall and reinstall the device or program to regain its original settings.
In all fairness, Vista isn’t the only version of Windows that experiences network glitches. XP has its own series of connectivity aggravations, as you can see by a search-engine query of Microsoft’s support center.

If your connectivity problems aren’t resolved by using the points discussed above, you may be suffering from an even more obscure issue. If so, ruling out the tricky configuration problems I describe here may at least help you isolate the real problem and restore your network link.

Scott Spanbauer writes frequently for PC World, Business 2.0, CIO, Forbes ASAP, and Fortune Small Business. He has contributed to several books and was technical reviewer of Jim Aspinwall’s PC Hacks.