Windows Tip: Resolving IP address conflicts
A basic but often hard to troubleshoot issue that can arise on a TCP/IP network is an IP address conflict. This occurs when two computers on a network are assigned the same IP address, and this situation can arise in three different ways. First, you might misconfigure your network by manually assigning two computers the same static IP address. This is a common occurrence on smaller networks especially when computers are in a workgroup not a domain. Second, a DHCP server on your network might assign an address to a client computer that conflicts with a static IP address assigned to some other network device such as a router or printer. And third, if you have two DHCP servers on the same subnet that have overlapping scopes, you're likely to get client computers with duplicate addresses.
What I've discovered is that troubleshooting these situations not only requires clear thinking but sharp eyesight as well. For example, say a Windows XP computer boots up and the user logs on and there's an address conflict with another computer on the network. In this circumstance, a caution icon should appear in the notification area together with a popup balloon saying "Warning-System Error: there is an IP address conflict with another system on the network." But say the user closes that popup without digesting it properly (those pesky users) and then calls the Help Desk and says "I can't connect to the server." Joe Support Desk tries Offer Remote Assistance but that doesn't work because there's no network connectivity with the user's computer. So Joe puts down his coffee and ambles over to the user's computer to try and find out what's going on.
Once he's there, Joe notices a funny thing about the network icon in the notification area -- it's got a wee little yellow light moving back and forth beneath it (see screenshot). This means your network connection is seeking a DHCP server that can assign it an address. You typically see this for wireless connections that are trying to establish a connection to an access point, but you rarely see it for a wired (LAN) connection -- unless your computer has a duplicate IP address with some other computer on the network.
This is one reason why I always like to pre-configure workstations so that the network icon is visible in System Tray. To do this, open the TCP/IP properties of the connection and on the General tab click Properties and select the checkbox labeled "Show icon in notification area when connected." But what if you haven't configured workstations this way? In that case, you can open a command prompt window and type ipconfig on the computer and if it displays 0.0.0.0 for both the computer's address and default gateway, you've likely got an address conflict with another computer. Just to be double sure, open Event Viewer next and check the System log for an Error event that has source Tcpip and event ID 4198.